Naked Science Forum
On the Lighter Side => That CAN'T be true! => Topic started by: gem on 07/08/2021 18:29:52
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Hi all,
So Colin has been sailing recently, So not only is he good at physics he will also have been experiencing natures laws and forces first hand.
Would anyone like to venture an answer.
:)
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Wind.
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Entropy.
A mass of air moving at one velocity, in close proximity to a mass of water moving at a different velocity.
Colin's sailing boat sits at the interface of these two masses - after the boat passes, the air is moving a bit slower, and the water is moving a bit faster. The difference in velocity between these two masses is less.
Colin's enjoyment of the world has increased, as has the entropy of the world.
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I read of some rather funny claim that it was due to the spin of the Earth, but it's generally going to be solar powered.
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I do hope he didn't make the mistake of using a flat sail when travelling crosswind.
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Hi all,
So Evan_au
A mass of air moving at one velocity, in close proximity to a mass of water moving at a different velocity.
Colin's sailing boat sits at the interface of these two masses - after the boat passes, the air is moving a bit slower, and the water is moving a bit faster. The difference in velocity between these two masses is less.
Colin's enjoyment of the world has increased, as has the entropy of the world.
Now lets not get to nautical, but i'm sure Colin may correct you on sailing and the importance of tide tables, especially if he's been sailing in
"The Solent" and going with the tidal current.
However if Origin and BC are correct wouldn't the entropy/transformation. be reversed the next day ?
Origin
Wind.
BC
but it's generally going to be solar powered.
:)
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Wind and tide may change but entropy doesn't generally get reversed. In fact the summary of life, the universe, and everything, is ΔS > 0.
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Hi all
yes thanks Alan, a poor use of word on my part, I probably should have said replenished.
So doesn't the continued increase of entropy of the Earth system due to Colin's sailing activities continually decrease also due to the solar input ?
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No.
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However if Origin and BC are correct wouldn't the entropy/transformation. be reversed the next day ?
The entropy change might be happening in the Sun...
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Hi all
Alan
No
Perfect, ;)
So we are agreed Colin's sailing boat actual physical processes that gives him his "speed over ground" are not symmetric in time ?
Such as the heeling over that occurs, when the wind billows his sails full and the tell tails go horizontal and the ropes groan under tension, similar to the flapping and rustling of foliage on dry land. :)
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You could look at a simpler model. The motion of a boat through moving water is depressingly complicated, but if we just look at the sail as an aerofoil you can see that wind can produce a force on the foil, and waving a sail can produce wind. Flapping sails aren't a lot of use, but propellors and windmills are simply time-reversals of the same Bernouilli effect.
Incidentally if BC is awake (does he ever sleep?) he might ponder why a boat can sail faster across the wind than it can downwind, if the underside of an aerofoil is the only surface that contributes lift.
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Hi all,
Alan in regards to the faster than the wind I believe there is a post already live on that issue, here on tns.(something about a bet was in the media I believe)
On the other point of reversibility and time symmetry I don’t want to get to in depth if possible but a general agreement as to the fact of heat input and friction are involved should do away with Bernoulli effect as a counter argument, as to what’s actually occurring in reality.
😊
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Hi all,
So lets assume as suggested the source of the kinetic energy is the Suns electromagnetic radiation and lets also assume a speed over ground of 4 m/s therefore for every kilogram mass of the boat we have
kinetic energy of 8.0 Joules and corresponding momentum of 4.0 Kg m/s
Now if we consider the energy and momentum force of the electromagnetic radiation received at the earths surface,
so for ease lets approximate the intensity at 1KW /m^2 that is 1000 J/s
and this gives a momentum pressure of between 3.33 x 10^-6 N/m^2 and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.
So why is there such disparity between the two sets of figures for the value of momentum ?
where did the mass of the boat get this momentum from.
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The earth has an awful lot of square meters compared with a boat.
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lets assume as suggested the source of the kinetic energy is the Suns electromagnetic radiation
I think that is a fair assumption.
Light delivers a lot of electromagnetic energy: much of which gets turned into heat, some of which appears as the kinetic energy of circulating air currents and ocean currents.
But then you jump to this:
this (momentum of light) gives a momentum pressure of between 3.33 x 10^-6 N/m^2 and 6.67 x 10^-6 N/m^2
As you observe, the pressure that photons deliver directly to the sail is small.
- That is why solar sails are only useful in space, where there is no friction, and no air currents.
The force of air pushing on the sails is significant, and the force of water pushing on the keel is significant
- But the force of photons pushing on the sails is insignificant
Colin's sailboat doesn't go anywhere when it is becalmed (no air currents), even when the Sun is still shining brightly.
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ncidentally if BC is awake (does he ever sleep?) he might ponder why a boat can sail faster across the wind than it can downwind, if the underside of an aerofoil is the only surface that contributes lift.
I did
https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643539;topicseen#msg643539
You can make the idea work with no classic "aerofoils" at all- you can use flat planks to build the propeller. It won't be efficient, but it doesn't need to be
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On my planet, a plank still has two sides.
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Hi.
and this gives a momentum pressure of between 3.33 x 10^-6 N/m^2 and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.
So why is there such disparity between the two sets of figures for the value of momentum ?
where did the mass of the boat get this momentum from.
Two things seem to be relevant:
1. Momentum is a vector quantity not a scalar. Conservation of momentum doesn't state that the total sum of the absolute value of momentum never increases, only that the vector sum of the momentum remains constant. This is a lot of words so I'll re-phrase it another way:
A big blob of matter might be at rest @@ but it can suddenly split into two smaller blobs of matter
←@ @→ where each individual little blob moves away from the other.
This is perfectly fine provided the sum of the momentum to the right → minus the momentum to the left ← still equals 0.
The absolute momentum has increased BUT the vector sum of momentum has not changed.
2. The atmosphere can undergo processes where momentum is created, provided the vector sum of momentum remains constant. As a consequence the absorption of energy from the radiation of the sun becomes as important as the absorption of the momentum from the radiation. Energy can be used to apply a force to some part of the atmosphere and keep that force applied over a certain distance.
Sailing is fine pursuit. I wish you fair winds and calm seas. Bye for now.
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Momentum is a vector quantity not a scalar.......the sum of the momentum to the right → minus the momentum to the left ← still equals 0.......The absolute momentum has increased
Something of an oxymoron. The sum of the arguments has increased, but as momentum is a vector, that has nothing to do with momentum! You have increased the kinetic energy (a scalar) of the system.
Beware - you could incur the wrath of a bored chemist as well as the raised eyebrow of a rather busy physicist.
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Hi all,
thanks for the responses,
Now below is my bold:
Evan_au states
Light delivers a lot of electromagnetic energy: much of which gets turned into heat, some of which appears as the kinetic energy of circulating air currents and ocean currents.
ES states;
The atmosphere can undergo processes where momentum is created, provided the vector sum of momentum remains constant. As a consequence the absorption of energy from the radiation of the sun
Now as Alan has highlighted these aspects are going to cause some disagreements, but it is an area that warrants inspection.
So in that regard I would like to start this aspect of the discussion off with, the spontaneous aspect of, convection and buoyancy from thermal expansion/contraction.
Being a potential source of the momentum of Colin's Sailing boat.
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On my planet, a plank still has two sides.
Well, if you like, you can sculpt it carefully into such a shape that, when you make a prop out of it, the thrust on it is the same on both faces.
And this will still work if you write "top" one one face and "bottom" on the other.
(Whichever way round you think they should be on your planet...)
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To save BC the bother of explaining it, here's what happens.
Sun heats earth, earth heats atmosphere. But the heating is uneven (due to the earth being spherical and the sun a long way away) so the atmosphere starts to move in fairly complex convection cells.
Because the atmosphere is rotating with the earth, bits moving away from the equator have more angular momentum than bits moving from the poles, so the convection cells have an east-west flow vector.
This is the principal source of motive power for ocean sailing. Lots to do with the considerable energy of solar photons, very little to do with their minuscule momentum.
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It's important to recognise that the Earth not only gets heated by the Sun, but that it's also cooled by radiation to the night sky.
It's the temperature difference which drives the heat engine which we call weather.
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this gives a momentum pressure of between 3.33 x 10^-6 N/m^2 and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.
The Crooke's Radiometer, or "light mill" lets you test this calculation.
If light is perfectly reflected (silvered side), you transfer twice the momentum as the absorption case (black side), which means that the silvered side will move away from the source of light:
In fact, the black side moves away from the source of light.
- This is because the photon carries electromagnetic energy, E=hf. This is absorbed by the black side, heats it up, which heats the adjacent air, and starts the light mill turning
- This force of heated air is greater than the momentum transferred by photons to the mirrored side.
The Light Mill is in a partial vacuum (about 0.1% of atmospheric pressure).
- If you put in in a very good vacuum, with very good bearings, the light pressure is the primary force, and it turns in the other direction.
See: https://en.wikipedia.org/wiki/Crookes_radiometer
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It's important to recognise that the Earth not only gets heated by the Sun, but that it's also cooled by radiation to the night sky.
It's the temperature difference which drives the heat engine which we call weather.
Except that it never gets dark at the north pole in summer, but it's still colder than the equator, where it's dark half the time.
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Hi all,
thank you Evan a very delicate piece of engineering
BC and Alan yes all very relevant points as to whats under consideration, we also have to consider the further expansion and consequence of cooling due to expansion as the warmed air rises and subsequent drop in pressure.
But these are all aspects of the dynamics occurring due to the solar input.
Just to put a approximate handle on the solar force due to the Suns rays impacting the cross sectional area at earths surface per sec at 1.0 kw/m^2 gives 399.6 x10^6 N
compared to the momentum force carried by a 10 metric tonnes of 40 ft sailing yacht traveling at 4 m/s of 40 x10^3 N
therefore the force of the suns light hitting earth is approximately = to 10.000 sailing yachts colliding with the earths surface/sec
or
considerably less than one very large crude oil carrier /sec
So in the scheme of things, "a drop in the ocean" when it comes to considering the momentum of the atmosphere, and the non conserved forces powering Colin's boat. as you say Alan it requires a motive force.
So where is this momentum coming from that Colin's able to harness ?
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The increased momentum of the moving boat comes from the reduction of momentum of the air around it, if we ignore the downwind and tidal drifts. Land yachts are a simpler model as they are not subject to tidal current and have negligible downwind drift.
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It's important to recognise that the Earth not only gets heated by the Sun, but that it's also cooled by radiation to the night sky.
It's the temperature difference which drives the heat engine which we call weather.
Except that it never gets dark at the north pole in summer, but it's still colder than the equator, where it's dark half the time.
You are right, sloppy thinking on my part. The cooling is due to radiation to bits of the sky that are not the Sun- whether it's night or day and that's the difference that drives things.
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Every point of the planet is illuminated by the sun for exactly half of the year.
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Every point of the planet is illuminated by the sun for exactly half of the year.
That's an idea you can stick where the sun doesn't shine.
However, some bits of the planet see it square on and some don't.
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Hi all,
So BC,
However, some bits of the planet see it square on and some don't.
yes that is very true, one way to demonstrate the relevance of this is with a torch on a flat surface and by changing the angle of incidence alters the area the power of the torch lights up, therefore altering the energy per unit area.
Now Alan yes I believe this is reasonable, assumption.
The increased momentum of the moving boat comes from the reduction of momentum of the air around it,
Also
To save BC the bother of explaining it, here's what happens.
Sun heats earth, earth heats atmosphere. But the heating is uneven (due to the earth being spherical and the sun a long way away) so the atmosphere starts to move in fairly complex convection cells.
Because the atmosphere is rotating with the earth, bits moving away from the equator have more angular momentum than bits moving from the poles, so the convection cells have an east-west flow vector.
This is the principal source of motive power for ocean sailing. Lots to do with the considerable energy of solar photons, very little to do with their minuscule momentum.
So yes this is also an aspect of the dynamics as demonstrated by the Coriolis force but I would say this is an effect not the cause of the dynamic.
So BC
It's important to recognise that the Earth not only gets heated by the Sun, but that it's also cooled by radiation to the night sky.
It's the temperature difference which drives the heat engine which we call weather.
Yes agreed the dynamical motion of the atmosphere is also dictated by change in pressure as well as change in density, as radiation pours in/out of the Earth system continuously altering the "internal energy/momentum"
( molecular translational energy.)
Therefore constantly changing the gradient of the pressure.
So I believe, we have agreed the Suns input has provided the energy for Colin's boat, I don't believe we have a consensus as to the source of Momentum his boat acquires, IE is it generated on earth due to the suns input to the condition's which allow this to occur or is it already in existence ?
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Now Alan yes I believe this is reasonable, assumption.
The increased momentum of the moving boat comes from the reduction of momentum of the air around it,
You both seem to be treating momentum like a scalar. Energy works this way, but not momentum, which is a vector. The system's total momentum cannot change, but the magnitude of the momentum of the parts can. That isn't conserved. Take the collision of a pair of clay wads. In some inertial frame, they each have equal and opposite momentum, but after the collision they're both stopped. Scalar momentum was not conserved in that case, but vector momentum was.
Short story: The input of energy from the sun aside, the momentum of the boat/air/water system does not change, but any momentum change of the boat must be countered with an equal an opposite change in total momentum of the air and water. If there's no momentum change of the boat (cruising at fixed speed), then there's no change in total momentum of the air and water.
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That's an idea you can stick where the sun doesn't shine.
On the dark side of Venus?
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You both seem to be treating momentum like a scalar.
not at all
the momentum of the boat/air/water system does not change, but any momentum change of the boat must be countered with an equal an opposite change in total momentum of the air and water. If there's no momentum change of the boat (cruising at fixed speed), then there's no change in total momentum of the air and water.
which is a longwinded version of what I wrote!
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Hi, Colin... you seem to be notably absent from this thread...
I hope we haven't offended you by using your hobby as a an object lesson in physics?
Or maybe you didn't want to disappoint us all by telling us that your boat actually runs on petrol/gasoline?
Can you finally resolve these questions for us, please?
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Hi all,
So Halc you appear to be missing out some of the realities of the physical world ! ( my bold )
The input of energy from the sun aside, the momentum of the boat/air/water system does not change, but any momentum change of the boat must be countered with an equal an opposite change in total momentum of the air and water. If there's no momentum change of the boat (cruising at fixed speed), then there's no change in total momentum of the air and water.
The sun does have input
In some inertial frame, they each have equal and opposite momentum, but after the collision they're both stopped. Scalar momentum was not conserved in that case, but vector momentum was.
So is the surface of the earth and its atmosphere an inertial reference frame ? if so why does convection occur ?
It is believed the collisions of air molecules between each other are elastic but with the surface of the earth/objects are varying degrees inelastic therefore not Isentropic.
Therefore ultimately heat can be generated from these interactions/dynamics and energy/momentum can be radiated out of the Earth system.
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Hi again.
Can you finally resolve these questions for us, please?
We're not that interested in where the sailing boat comes from just where the momentum and energy comes from. However, you already know answer Evan_au. Even if it is petrol powered, that is still energy (but not momentum) from the sun's radiation a few years ago.
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Gem seems to have written something while I was writing this:
You said "hi all" so I'm guessing you aren't specifically targetting Halc.
So is the surface of the earth and its atmosphere an inertial reference frame ? if so why does convection occur ?
A reference frame fixed to the surface of the earth isn't a perfectly inertial reference frame but it's good enough for most purposes.
Convection --> Is a net flow of fluid that effectively carries heat from one place to another. It is a consequence of different densities in different regions of a fluid and requires a gravitational field or similar acceleration to explain it. Near the surface of the earth, a hot region of fluid tends to rise upward while colder regions of fluid tend to fall downward. (More information available at: https://en.wikipedia.org/wiki/Convection - but I expect you already know that).
It is believed the collisions of air molecules between each other are elastic but with the surface of the earth/objects are varying degrees inelastic therefore not Isentropic.
Yes, they may be inelastic. However, many simple models assume elastic collisions when describing ideal fluids and the results are good enough for many things. I'm not sure why inelastic collisions would prevent isotropy. There's no preferred direction for anything (at least not until gravity is introduced into the fluid dynamics).
Therefore ultimately heat can be generated from these interactions/dynamics and energy/momentum can be radiated out of the Earth system.
Seems right. Energy in the form of infrared radiation does get emitted by earth and it's atmosphere and sent into space. Roughly the same total energy is radiated out into space as that which is absorbed from space, otherwise the planet would quickly heat up. I thought the rough count was that for every 1 typically high energy photon coming in from the sun and absorbed by the earth about 3 lower energy photons (each with about one-third the energy of the incoming photon) are re-radiated by the earth back out into space. (This is, at best, an average and more detailed information is available elsewhere).
Best Wishes.
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You both seem to be treating momentum like a scalar.
not at all
the momentum of the boat/air/water system does not change, but any momentum change of the boat must be countered with an equal an opposite change in total momentum of the air and water. If there's no momentum change of the boat (cruising at fixed speed), then there's no change in total momentum of the air and water.
which is a longwinded version of what I wrote!
I was commenting on the line you edited out:
The increased momentum of the moving boat comes from the reduction of momentum of the air around it,
In the proper frame of the system in question, if the boat momentum increases, then so must the combined momentum of the air/water with which it interacts, but you say it reduces. This might result in conservation of scalar momentum (something that is not conserved), but not conservation vector momentum, which is why I accused you of treating it like a scalar.
Conservation of momentum demands that if one goes up, so does the other, at least in the proper frame of the closed system.
The real system isn't closed. No matter how one delimits a system, there are external forces acting on it, but I'm discussing the ideal simplified case.
gem, I have issues with some of your terminology which makes some of your posts meaningless.
compared to the momentum force
Momentum isn't a force. Different units.
carried by a 10 metric tonnes of 40 ft sailing yacht traveling at 4 m/s of 40 x10^3 N
The yacht carries no force. If it isn't accelerating, then the net force exerted by (or on) it is zero. 10,000 kg * 4 = 40,000 kg•m/s, not 40,000 N. Getting the units right is half the story. The force exerted on Earth by solar wind/light seems pretty irrelevant to this since it is nicely balanced by gravity in the opposite direction. The sun's direct contribution is energy, not momentum.
The force exerted by your 40t yacht on collision depends on how much it crumples on impact.
So where is this momentum coming from that Colin's able to harness ?
Again, if the boat isn't accelerating, then no momentum transfer at all is needed to keep it thus. It just needs energy input to maintain the speed.
The sun does have input
I didn't say otherwise. I just ignored it since it has no immediate direct effect, else sailing would be impossible at night. Broken down to simple requirements, you have water, air moving relative to the water, and the boat (A, W, B). That's enough. Any more is needless complication.
In some inertial frame, they each have equal and opposite momentum, but after the collision they're both stopped. Scalar momentum was not conserved in that case, but vector momentum was.
So is the surface of the earth and its atmosphere an inertial reference frame ?
I made no mention of the surface of Earth there. I referred to the proper frame of the two clay wads, which must exist.
I kind of presumed no angular momentum in that system. Linear momentum is something that cannot be bottled, but interestingly, angular momentum can. So reactionless thrust is impossible, but reactionless rotation is not. The Hubble telescope is a great example of reactionless rotation and the bottling of angular momentum. I digress...
If we break our sailing experience into a closed system with a body of air and water, then in some frame the sum of the momentum of W+A+B = 0. That's the frame I'm talking about, and it probably isn't the local frame of the surface of Earth. The entire surface of Earth is rotating and thus cannot define an inertial reference frame.
if so why does convection occur ?
Irrelevant to why the boat sails through the water. Convection occurs because the sun heats stuff, making it move around, which explains why there's wind, but not how the wind makes the boat go. In the simplified example, there's just a massive wad of air moving with its own inertia. It's a closed system, so no input of any kind.
Therefore ultimately heat can be generated from these interactions/dynamics and energy/momentum can be radiated out of the Earth system.
Yes, sailing is going to involve expenditure of energy, which turns to heat. Things will slow down without new energy input. But my air wad and water wad is big so it doesn't happen quickly. Heat may radiate away, but momentum doesn't
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Hi, Colin... you seem to be notably absent from this thread...
Limited access to wifi and reliable phone signal plus before getting underway every none essential eg ipad, laptop, is secured so it doesn’t fly around.
I hope we haven't offended you by using your hobby as a an object lesson in physics?
not at all, but not sure it is essential to what gem is looking for
Or maybe you didn't want to disappoint us all by telling us that your boat actually runs on petrol/gasoline?
sometimes, small engine for close quarters manoeuvres and as an ‘iron sail’ in no wind.
Can you finally resolve these questions for us, please?
If they were really sailing questions, then yes, but I think that is a side issue for gem.
There have been some comments about the mechanics of sailing, some right, some not so right. Happy to go into those if anyone is interested, but as Alan says it’s complicated. So, I tend to consider the boat as a system with external forces, eg wind, current (tide), hull wing effect, rudder etc, and the trick is to balance all those in order to get where we want - without resorting to the iron sail.
However, I don’t think that’s what gem is after. I think he wants to prove that momentum is not conserved in the earth atmospheric system, but I haven’t yet seen him clearly define what the closed system is that he wants to consider, perhaps I’ve missed it.
Anyway, must get everything battened down and go over today’s passage plan.
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Anyway, must get everything battened down and go over today’s passage plan.
And roughly where are you sailing, if you don't mind telling us?
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Hi all,
So Halc,
The yacht carries no force. If it isn't accelerating, then the net force exerted by (or on) it is zero. 10,000 kg * 4 = 40,000 kg•m/s, not 40,000 N. Getting the units right is half the story. The force exerted on Earth by solar wind/light seems pretty irrelevant to this since it is nicely balanced by gravity in the opposite direction. The sun's direct contribution is energy, not momentum.
The force exerted by your 40t yacht on collision depends on how much it crumples on impact.
If you look back I used the correct units of momentum, and when putting the argument as to help scale the implications as to the equivalence of this, the continual force of the radiation pressure applied to the cross sectional area of the Earth
rather than just Numbers to powers of 10 that many would struggle to put a real life value to.
( F = radiation pressure x area ) units:Newtons
which I gave an example of being less than one super tanker continually colliding spread across the CSA.
Again, if the boat isn't accelerating, then no momentum transfer at all is needed to keep it thus. It just needs energy input to maintain the speed.
What form of energy is required for a sailing boat ?
kinetic energy = 1/2 x mass x velocity^2 momentum = mass x velocity, difficult to separate them, so to keep the boat at a constant velocity a constant momentum transfer from the atmosphere is required.
To dismiss convection you state Halc ; my bold
Irrelevant to why the boat sails through the water. Convection occurs because the sun heats stuff, making it move around, which explains why there's wind, but not how the wind makes the boat go. In the simplified example, there's just a massive wad of air moving with its own inertia. It's a closed system, so no input of any kind.
I think you just made quite a good case for the source of Colin's sailing boats, momentum. especially as we know it isnt a closed system and there is an input.
Also Halc, I believe BC already covered the day/night aspect and the Suns input you are happy to put to one side.
I just ignored it since it has no immediate direct effect, else sailing would be impossible at night.
I
Heat may radiate away, but momentum doesn't
I'm afraid that's not true either, I believe ES covered aspects of that.
Colin I hope your having a great time.
However, I don’t think that’s what gem is after. I think he wants to prove that momentum is not conserved in the earth atmospheric system, but I haven’t yet seen him clearly define what the closed system is that he wants to consider, perhaps I’ve missed it.
Yes your partially correct because I don't see a closed system.
However this is only a preamble that links to several other phenomena,
I suppose current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
Given the motion/momentum generated, impacts things like sail boats mountains trees. and oceans, with varying degrees of inelastic collisions that would alter the vector total expected from a closed system.
because the solid earth cannot discriminate between these collision and a meteorite, in regards to the potential change of momentum.
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You are repeating the same mistakes, so it appears that your goal is to assert nonsense rather than learn anything.
My contribution to the thread is about how wind makes a sailboat go and not about how how much momentum of Earth is or isn't preserved in the presence of the sun.
Earth is a system, but hardly a closed one. There is very much energy continuously going in and out of Earth. Momentum is constantly changing. Mass is even changing. Nobody claims it is a closed system. Hence my terse hand-wave that the radiance of the sun is primarily responsible for wind.
There is also significant wind on Jupiter, which is not mostly due to the sun's radiance, so wind very much can form in a closed system.
If you look back I used the correct units of momentum
10 metric tonnes of 40 ft sailing yacht traveling at 4 m/s of 40 x10^3 N
You specified Newtons, which is mass•distance/sec² whereas momentum is mass•distance/sec.
A sailboat moving at a constant 4 m/sec has zero acceleration. Remember F=ma, taught when you were 12? If 'a' is zero, so is F. If acceleration is zero, then velocity is constant, and therefore zero momentum transfer from the boat to the rest of the system.
What form of energy is required for a sailing boat ?
kinetic energy = 1/2 x mass x velocity^2 momentum = mass x velocity, difficult to separate them
They're very separate things. Difficult to combine them, but you manage to do it, with the resulting nonsense. But KE is probably inappropriate since the boat is neither gaining nor losing KE, just as it is neither gaining nor losing momentum.
So the way to analyze it is to demonstrate that the forces acting on it must add up to zero. Those forces, individually, represent work, not KE. So the form of energy required for a sailing boat is better expressed as work done on it.
to keep the boat at a constant velocity a constant momentum transfer from the atmosphere is required.
But no momentum transfer to or from the boat, whose momentum is constant since both its mass and velocity are constant. So any momentum transferred to the atmosphere must come from somewhere other than the boat.
momentum. especially as we know it isnt a closed system and there is an input.
'Momentum' doesn't define a system, closed or otherwise.
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Hi all,
Halc
Please take a look at post 13 I believe the units are correct given:
The longer the electromagnetic wave is incident on the object, the more momentum is transferred. This time dependence complicates matters, though, so it can be defined where it does not depend on time, this is called radiation pressure, N/m^2
Halc
Remember F=ma, taught when you were 12? If 'a' is zero, so is F. If acceleration is zero,
Maybe you should have put net into the point
Also can be written as F = rate of change of momentum !
Halc
But no momentum transfer to or from the boat, whose momentum is constant since both its mass and velocity are constant. So any momentum transferred to the atmosphere must come from somewhere other than the boat
Now Halc we are considering a dynamical real world interaction, where there is a continuous exchange of energy and momentum to allow a sailing boat to maintain a constant velocity through the fluids, Indeed friction is a consequence of the momentum being transferred, in these inelastic therefore not Isentropic interactions
Halc what you did here by misquoting me, I believe is disingenuous and doesn't reflect well on you, at least you left the full stop in. :(
(gem quoted by Halc) (momentum. especially as we know it isnt a closed system and there is an input.)
Halcs comment
'Momentum' doesn't define a system, closed or otherwise.
Now I believe the pertinent point to be addressed is:
gem
I suppose current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
Given the motion/momentum generated, impacts things like sail boats mountains trees. and oceans, with varying degrees of inelastic collisions that would alter the vector total expected from a closed system.
because the solid earth cannot discriminate between these collision and a meteorite, in regards to the potential change of momentum.
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Please take a look at post 13 I believe the units are correct given
OK. I admit I probably had never read post 13. I can comment on that if you like.
So lets assume as suggested the source of the kinetic energy is the Suns electromagnetic radiation
I agree that the majority of the energy driving the wind comes from this energy source, but EM radiation from the sun is not in the form of KE. It gets translated to that in a complicated and chaotic process.
assume a speed over ground of 4 m/s therefore for every kilogram mass of the boat we have
kinetic energy of 8.0 Joules and corresponding momentum of 4.0 Kg m/s
That is correct units, yes. Not sure why you changed it to force in a later post.
... this gives a momentum pressure of between 3.33 x 10^-6 N/m^2 and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.
That pressure is countered by a much larger force in the opposite direction which is why Earth acceleration vector is towards the sun and not away from it. After a full year, the Earth is right were it was before and at the same velocity, so this continuous force (by radiation pressure, gravity, other effects) has no cumulative effect on the KE or momentum of Earth It isn't the cause of the wind, whereas the radiant energy is, but that also is the source of (most of) the eventual KE energy of my car, not just the sailboat. Likewise, the continuous 4 million N force on the ISS adds no KE to it. This is why I suggest analyzing the effects of force in terms of work.
where did the mass of the boat get this momentum from.
If it's moving at a constant 4 m/s, it already has it. The source is itself. You don't seem to be considering the case where it gets going from a stop, but even then, the source of momentum is pretty obvious, and it isn't the sun at all.
Maybe you should have put net into the point
Also can be written as F = rate of change of momentum !
OK, net F = rate of change of momentum. And net F on the sailboat is nada, so no momentum change.
But no momentum transfer to or from the boat, whose momentum is constant since both its mass and velocity are constant. So any momentum transferred to the atmosphere must come from somewhere other than the boat
Now Halc we are considering a dynamical real world interaction, where there is a continuous exchange of energy and momentum to allow a sailing boat to maintain a constant velocity through the fluids
There might be a net exchange of energy and momentum going on, but neither net exchange is with the boat which has zero change in both. You seem to both assert this but then not realize it.
Indeed friction is a consequence of the momentum being transferred, in these inelastic therefore not Isentropic interactions
OK, but I was not discussing entropy.
Halc what you did here by misquoting me, I believe is disingenuous and doesn't reflect well on you, at least you left the full stop in.
OK. It appeared to me that you were using the word momentum as a description of a system, in this statement:
I think you just made quite a good case for the source of Colin's sailing boats, momentum. especially as we know it isnt a closed system and there is an input.
Both 'sentences' admittedly do not parse, and I interpreted it incorrectly. No clue what you meant by the two fragments, neither of which is a well formed sentence. The word 'momentum' doesn't seem to fit in at all with any of it.
If you're saying the boat isn't a closed system, then we all agree with that.
I suppose current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
This is complicated, but the basics are not hard to comprehend. It involves heat, buoyancy, friction, and Coriolis effects. The model allows Earth to be approximated as a closed system to momentum in its own frame, especially if we remove tidal effects, which is a very real and continuous external torque on the system. External things like meteors have negligible effect since they for the most part cancel each other out. Of course it cannot be closed to energy.
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Hi all
Ok lets focus on what I posted as to what I believe is pertinent, for current theory to remain.
gem
I suppose current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
Given the motion/momentum generated, impacts things like sail boats mountains trees. and oceans, with varying degrees of inelastic collisions that would alter the vector total expected from a closed system.
because the solid earth cannot discriminate between these collision and a meteorite, in regards to the potential change of momentum.
Halc
This is complicated, but the basics are not hard to comprehend. It involves heat, buoyancy, friction, and Coriolis effects. The model allows Earth to be approximated as a closed system to momentum in its own frame, especially if we remove tidal effects, which is a very real and continuous external torque on the system. External things like meteors have negligible effect since they for the most part cancel each other out. Of course it cannot be closed to energy.
I agree the dynamics are complicated, so lets bring it back to the basic points I made in the statement above,
where is the action reaction pairing that causes the fluid flow that occurs spontaneously actually occurring ?
And does that location allow the solid earth to discriminate between these consequential collisions and a meteorite's collision, in regards to the potential change of momentum ?
If it cannot be categorically stated otherwise then we will have to look at the questions this raises, for example appearance of neutrality/canceling of the effect of these frictional forces as Halc attributed to the meteors collisions, also external torque forces, and apparent stability of length of day.
For example can the knowledge of the known external torques predict specific dates the earth rotation will be slower/faster than the day before/after ?
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I suppose current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
Not sure what you mean by this
For example can the knowledge of the known external torques predict specific dates the earth rotation will be slower/faster than the day before/after ?
Not sure what external torques you mean, but if you are interested in rotational variability this is the go to guy, done a lot of work on it, but I’m not sure about forecasts because of the huge variations involved. This is a layman intro:
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
By the way, note that this is due to the transfer of momentum, not the creation of momentum, due to interaction at earth/atmosphere boundary etc.
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A similar question for landlubbers about "where wind force, energy and momentum come from" got turned into a $10,000 bet:
See (18 minutes):
PS: Veritassium is a good series of sciency Youtube videos...
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A similar question for landlubbers about "where wind force, energy and momentum come from" got turned into a $10,000 bet:
I did notice there was a thread on this but I didn’t have time to follow it through in detail.
The centre of effort is a bit high for a boat, which has triangular sails to keep the centre of effort low, otherwise you get a lot of heeling and reduced effective sail area. I suspect a boat would also start to dig her nose in when on a run.
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Hi all,
thanks for the link Colin, you said you were not to sure of a couple of points I made, below is an extract from the paper you linked to.
D. SALSTEIN
my bold
ABSTRACT. It was noted some time ago that the angular momentum of the atmosphere varies,
both regionally as well as in total. Given the conservation of angular momentum in the Earth system,
except for known external torques, such variability implies transfer of the angular momentum across
the atmosphere’s lower boundary. As nearly all is absorbed by the Earth below, the solid Earth
changes its overall rotation from this impact
So for conservation of the angular momentum of the earth system to underpin the current theory.
I believe the current theory needs to state, where the action reaction pairing occurs due to the spontaneous convection occurring in the earths uncoupled atmosphere.
Given the motion/momentum generated, impacts things like sail boats mountains trees. and oceans, with varying degrees of inelastic collisions that would alter the vector total expected from a closed/conserved system.
because the solid earth cannot discriminate between these collision and a meteorite, in regards to the potential change of momentum, if the initial, action reaction occurs independently of the earths surface/crust.
Also Colin you state;
By the way, note that this is due to the transfer of momentum, not the creation of momentum, due to interaction at earth/atmosphere boundary etc.
Do you believe these transfers are fully elastic or fully inelastic with the surface /atmosphere ?
Do you believe you utilized any of the earths conserved systems angular momentum with your sailing boat ?
Does convection alter the velocity of the atmosphere ?
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Do you believe you utilized any of the earths conserved systems angular momentum with your sailing boat ?
From the point of view of the paper I linked to the boat is part of the earth/atmosphere system.
Does convection alter the velocity of the atmosphere ?
Velocity yes, but that’s not implying a change of total momentum. Remember what goes up comes down in atmospheric circulation.
If we fill a container with water and heat a bottom corner water will rise up one side and circulate in the container, if that container is suspended freely it will rotate in the opposite direction conserving the original 0 angular momentum. If the container is in contact with the earth the total momentum will remain 0 as it will transfer momentum to the earth, but I doubt the earth notices.
My own interest in the earth/atmosphere interface is only that the coefficient of friction varies over the earth surface which leads to wind shifts due to a combination of pressure systems, laminar flow and Coriolis effect. As you might guess I’m interested in predicting those effects locally.
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Hi all,
OK lets run with that analogy,
Colin
If we fill a container with water and heat a bottom corner water will rise up one side and circulate in the container, if that container is suspended freely it will rotate in the opposite direction conserving the original 0 angular momentum
First I would like to check I understand your description, if a fluid circulates in vessel, are you saying the vessel will rotate in the opposite direction ? for example if I set a fluid(water) rotating in a container half full of water and then sat it in a body of water the vessel would naturally rotate in the opposite direction ?
???
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Hi all,
Also given the Coriolis effect in the atmosphere transporting the momentum that was once a property of the solid earth, how is this momentum perfectly transferred back, to the solid earth as measured by LOD, and not transferred to internal momentum of the elements that make up the fluid of the atmosphere.
Also How is this internal momentum of the atmosphere different from the internal momentum caused by the suns radiation.
Also where does the momentum of sound come from and go to ?
why did the LOD get shorter yesterday will today and tomorrow ?
:)
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First I would like to check I understand your description, if a fluid circulates in vessel, are you saying the vessel will rotate in the opposite direction ?
If 1) the vessel/water system had zero angular momentum before the heat was applied, and 2) no external torque is applied, then yes, rotation of the water one way must rotate the vessel the other way to maintain zero angular momentum. You can't change the total to a different number.
for example if I set a fluid(water) rotating in a container half full of water and then sat it in a body of water, the vessel would naturally rotate in the opposite direction ?
Sounds like the water already had angular momentum to start with in this case. No, the vessel has no reason to then rotate the other way.
If the vessel/water system was all stopped, and then a little internal propeller attached to the vessel started to rotate the water clockwise, then yes, the vessel would rotate counterclockwise due to the equal/opposite reaction of the propeller thingy attached to it.
Also given the Coriolis effect in the atmosphere transporting the momentum that was once a property of the solid earth
Coriolis effect has no direct effect on the total momentum of anything. Coriolis effects in the atmosphere is neither caused by nor causes any net force on Earth. If Earth (the interface between ground and air) was perfectly frictionless, Coriolis effect would still make nice hurricanes with distinct eyes and such. Probably more so since said friction lessens the severity of such storms.
Also How is this internal momentum of the atmosphere different from the internal momentum caused by the suns radiation.
One is (considerable) momentum (mostly angular) already a property of a system (atmosphere) and the other is a mild transfer (almost none of which is angular) which nets pretty much zero momentum over a year.
Also where does the momentum of sound come from and go to ?
Sound doesn't have significant momentum. A shock wave does. Regular ocean waves don't push on the continents more than still water does, but tsunamis do. The latter is a shock wave.
why did the LOD get shorter yesterday will today and tomorrow ?
It probably got longer, but the day to day variations can go either way, probably not due to any forces applied by the atmosphere. It definitely gets longer over time. It used to be well under 10 hours.
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Hi all,
Ok Halc to cover those points.
Halc
If 1) the vessel/water system had zero angular momentum before the heat was applied, and 2) no external torque is applied, then yes, rotation of the water one way must rotate the vessel the other way to maintain zero angular momentum. You can't change the total to a different number.
I'm not sure this is achievable, in reality couldnt it be argued the vector sum is zero if they all/both go round in the same direction ?
Halc
Sounds like the water already had angular momentum to start with in this case. No, the vessel has no reason to then rotate the other way.
If the vessel/water system was all stopped, and then a little internal propeller attached to the vessel started to rotate the water clockwise, then yes, the vessel would rotate counterclockwise due to the equal/opposite reaction of the propeller thingy attached to it.
The internal propeller attached to the vessel sounds suspiciously a bit like a boat and does not replicate the action reaction pairing of convection.
Halc
Coriolis effect has no direct effect on the total momentum of anything. Coriolis effects in the atmosphere is neither caused by nor causes any net force on Earth. If Earth (the interface between ground and air) was perfectly frictionless, Coriolis effect would still make nice hurricanes with distinct eyes and such. Probably more so since said friction lessens the severity of such storms.
Yes as I said momentum transported and transferred, but is it a 100% exchange rate ? is there the consequence of transfer to momentum of the molecules and bonds thereof ? (internal momentum )
Which is the context of the point made previously which the following doesn't address.
Halc
One is (considerable) momentum (mostly angular) already a property of a system (atmosphere) and the other is a mild transfer (almost none of which is angular) which nets pretty much zero momentum over a year.
Halc
. Sound doesn't have significant momentum. A shock wave does. Regular ocean waves don't push on the continents more than still water does, but tsunamis do. The latter is a shock wave.
wave momentum =energy/phase velocity so not sure how that fits with your statement ?
It probably got longer, but the day to day variations can go either way, probably not due to any forces applied by the atmosphere. It definitely gets longer over time. It used to be well under 10 hours.
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
You are disagreeing with the go to guy according to Colin, regarding the predictions to LOD I will post the results once they are published.
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Thus I must throw a wrench into the statement from @Colin2B :
If we fill a container with water and heat a bottom corner water will rise up one side and circulate in the container, if that container is suspended freely it will rotate in the opposite direction conserving the original 0 angular momentum.
While I agree, I don't think it is possible for the container to be 'suspended freely'. It can be done in zero g of course, but in that case, heating part of it isn't going to result in rotation of the liquid.
You beat me to the spanner @Halc
Almost immediately after posting it I began to wonder ‘how’, but let it stand while thinking through.
I dismissed putting a fan inside as that is a clear force against the container. There is the added problem that with a fan the water density distribution stays constant whereas in a heated system it does not so the centre of mass changes so a gimbal at the original centre of mass won't work.
It’s also not possible (practically) to have a frictionless vessel/water interface, so water motion should pull the container around; and as you say, finding a way to suspend eg a sphere, without friction is difficult.
I’m still thinking about it :)
I originally was going to suggest the jar as a model of a Hadley cell.
The only suggestion I have seen of sun heat increasing earth/atmosphere angular momentum was in a paper on extreme weather events and the effect on earth rotation. The discussion was around the effect of greater CO2 leading to a change of mass distribution in the atmosphere.
All other changes to earth rotation are due to friction forces, and pressure differentials across mountain ranges transferring momentum from atmosphere and ‘tuther way round.
EDIT:
You are disagreeing with the go to guy according to Colin, regarding the predictions to LOD I will post the results once they are published.
I’m not an expert in this area, my interest is strictly amateur although I’ve taken a number of courses in meteorology and sat in on some university courses. So I’m working on recommendations, and he does seem to be backing his work with actual measurements from satellite data etc.
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Hi all,
Halc / Colin, thank you for your candor,
In regards to the link you provided Colin, I would comment it is probably the best précis of current theory I have read.
However when you state your not an expert in this field I believe an inspection of the fundamentals that underpin current theory as stated in the abstract will be sufficient.
was noted some time ago that the angular momentum of the atmosphere varies,
both regionally as well as in total. Given the conservation of angular momentum in the Earth system,
except for known external torques, such variability implies transfer of the angular momentum across
the atmosphere’s lower boundary.
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
Halc I would comment on the bonds between molecules as being an aspect of freedom of movement (vibration rotation and hydrogen bonds ) which will be a factor of density change when energy is gained/lost.
On the scenario of the rotation of the container needing to go the opposite way to conserve momentum of the system, there are probably experiments that could be done to test the principle, but it will come back to my earlier comments, of the action reaction pairing.
In regards to the detectability of the frictional interactions I would point you to the link provided.
If momentum is generated in the atmosphere independently of the solid earth then the current theory is flawed given the subsequent frictional transfers.
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Hi all,
Halc
I think your skipping about a bit with this, it appears on one hand your saying an atmosphere can have momentum from energy input/electromagnetic radiation due to convection, where none previously existed.
Then also stating that violates basic physics.
Anyway as stated earlier predictions for LOD
gem;on: 17/08/2021 23:57:41
why did the LOD get shorter yesterday will today and tomorrow ?
Length of day variations/fluctuations in milli seconds as published today for the relevant dates. ( - = minus )
15/08/2021 - 0.4058
16/08/2021 - 0.6633
17/08/2021 - 0.9998
18/08/2021 - 1.0715
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I think your skipping about a bit with this, it appears on one hand your saying an atmosphere can have momentum from energy input/electromagnetic radiation due to convection, where none previously existed.
Yea, if it transfers momentum from something else.
Then also stating that violates basic physics.
No, I said your statement (momentum change without transfer from something else) is the violation.
Anyway as stated earlier predictions for LOD
gem;on: 17/08/2021 23:57:41
why did the LOD get shorter yesterday will today and tomorrow ?
Length of day variations/fluctuations in milli seconds as published today for the relevant dates. ( - = minus )
15/08/2021 - 0.4058
16/08/2021 - 0.6633
17/08/2021 - 0.9998
18/08/2021 - 1.0715
These are length of solar day, which get shorter this time of every year due to our orbit not being perfectly circular. This is purely a visual effect, not an actual delta in the spin rate.
I was talking about the spin rate, the sidereal rate, the time it takes to revolve exactly 360°, which is the rate from which the angular momentum is computed. This is a thread about momentum, not about when the sun appears to rise.
We both should have been more clear it seems.
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Hi all,
Ok to address your point;
Halc
These are length of solar day, which get shorter this time of every year due to our orbit not being perfectly circular. This is purely a visual effect, not an actual delta in the spin rate.
I was talking about the spin rate, the sidereal rate, the time it takes to revolve exactly 360°, which is the rate from which the angular momentum is computed. This is a thread about momentum, not about when the sun appears to rise.
We both should have been more clear it seems.
I'm afraid your mistaken, for example, the earth rotation ( LOD ) was actually slowing down between the 5th to the 13 th of august,
and the published values I posted are from :
LOD - FINALS.DAILY (IAU1980) from the IERS
So I repeat my previous post.
gem
why did the LOD get shorter yesterday will today and tomorrow ?
Length of day variations/fluctuations in milli seconds as published today for the relevant dates. ( - = minus )
15/08/2021 - 0.4058
16/08/2021 - 0.6633
17/08/2021 - 0.9998
18/08/2021 - 1.0715
-
Hi all,
Ok Halc it seems your getting a little frustrated and you seem to have a problem with the data.
These are length of solar day, which get shorter this time of every year due to our orbit not being perfectly circular. This is purely a visual effect, not an actual delta in the spin rate.
I was talking about the spin rate, the sidereal rate, the time it takes to revolve exactly 360°, which is the rate from which the angular momentum is computed. This is a thread about momentum, not about when the sun appears to rise.
I'm afraid your mistaken, for example, the earth rotation ( LOD ) was actually slowing down between the 5th to the 13 th of august,
and the published values I posted are from : LOD - FINALS.DAILY (IAU1980) from the IERS
What part of my comment is mistaken? I didn't contradict the numbers. I said that the LOD numbers are a visual effect, completely unrelated to the daily variations in the angular velocity of Earth. Do you have any reading comprehension skills at all???
why did the LOD get shorter yesterday will today and tomorrow ?
What part of my explanation cannot you comprehend?
Sorry, I cannot add more because you're just repeating things. The LOD variations as published on your site is due to our elliptical orbit and has nothing to do with changes in the time taken to go around 360°, which is about 23 hours 56 minutes with a lot less variation than the numbers you post, but variation nevertheless, and almost always increasing due to the perpetual negative torque continuously being applied from outside the planet.
Now the science that is behind the figures I posted is very comprehensive for the LOD,
derived from the EOP (Earth Orientation Parameters) determined by combining the most recently available observed and modeled data (including VLBI 24-hour and intensive, GPS, and AAM). The combination process involves applying systematic corrections and slightly smoothing, in order to remove the high frequency noise.
finals.daily from where I sourced the values, contains for the last 90 days the values from IERS Bulletin A for x/y pole, UT1-UTC, LOD, dPsi, dEps
Celestial pole offsets (dPsi, dEps) are related to the IAU1980 nutation theory.
Now EOP are the parameters which provide the rotation of the ITRS (The International Terrestrial Reference System) the ICRS (The International Celestial Reference System) as a function of time.
The ICRS is realized by VLBI estimates of equatorial coordinates of a set of extragalactic compact radio sources,
(the International Celestial Reference Frame)
Therefore I don't understand why you think the there should be a lot less variation than the numbers posted.
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Hi everyone.
Sidereal day vs. Solar day
This video draws the usual diagrams and says the usual things to explain the difference. You'll only need the first 2 minutes but the whole thing is only 3 minutes.
Now imagine the orbit of the earth around the sun is elliptical and not circular. The angle between the earth and the sun changes by a variable amount, it's not exactly 1 degree per day it just averages at about this amount.
Halc was suggesting your (Gem) data measures the solar day not the sidereal day.
Best Wishes.
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The difference between the apparent (based on the Sun) and real rotations of the Earth is this
https://en.wikipedia.org/wiki/Equation_of_time
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Hi all,
Thanks for the input ES and BC,
Yes its absolutely fair enough, Halc was making that point, and I thought I had addressed it by pointing out the reverse affect to the data I posted was measured to be happening a few days earlier, which wouldn't tie in with the Solar day example/argument, indeed its reversed again since I posted the results.
Also in regards to the value of the numbers involved I believe the range of the Solar day effect, is somewhere in the range/vicinity of ;
± 4 seconds throughout the year,
Whereas the fluctuations I posted sit within range of about
± 1.5 x10^-3 seconds throughout the year.
It really is quite an achievement of science that there is monitoring to that degree.
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Hi all,
So Halc,
These are length of solar day, which get shorter this time of every year due to our orbit not being perfectly circular. This is purely a visual effect, not an actual delta in the spin rate.
I was talking about the spin rate, the sidereal rate, the time it takes to revolve exactly 360°, which is the rate from which the angular momentum is computed. This is a thread about momentum, not about when the sun appears to rise.
I'm afraid your mistaken, for example, the earth rotation ( LOD ) was actually slowing down between the 5th to the 13 th of august,
and the published values I posted are from : LOD - FINALS.DAILY (IAU1980) from the IERS
What part of my comment is mistaken? I didn't contradict the numbers. I said that the LOD numbers are a visual effect, completely unrelated to the daily variations in the angular velocity of Earth. Do you have any reading comprehension skills at all???
why did the LOD get shorter yesterday will today and tomorrow ?
What part of my explanation cannot you comprehend?
Sorry, I cannot add more because you're just repeating things. The LOD variations as published on your site is due to our elliptical orbit and has nothing to do with changes in the time taken to go around 360°, which is about 23 hours 56 minutes with a lot less variation than the numbers you post, but variation nevertheless, and almost always increasing due to the perpetual negative torque continuously being applied from outside the planet.
Ok, given the last few posts have hopefully given a bit more clarity to the relevance to momentum of the data posted, are we ready to move past the comments above, or do you require me to repeat some points :)
As you will see the fluctuation range under discussion demonstrated in the link also
https://commons.wikimedia.org/wiki/File:Deviation_of_day_length_from_SI_day.svg#/media/File:Deviation_of_day_length_from_SI_day.svg
What Halc was mistaken by is a totally separate effect, as it states at the Top of the page, link below :
For regular changes in the length of a solar day through the year due to the obliquity of the ecliptic and the eccentricity of the Earth's orbit around the Sun, see Equation of time.
https://en.wikipedia.org/wiki/Day_length_fluctuations
Comprendi ?
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Hi all,
Thanks for the input ES and BC,
Yes its absolutely fair enough, Halc was making that point, and I thought I had addressed it by pointing out the reverse affect to the data I posted was measured to be happening a few days earlier, which wouldn't tie in with the Solar day example/argument, indeed its reversed again since I posted the results.
Also in regards to the value of the numbers involved I believe the range of the Solar day effect, is somewhere in the range/vicinity of ;
± 4 seconds throughout the year,
Whereas the fluctuations I posted sit within range of about
± 1.5 x10^-3 seconds throughout the year.
It really is quite an achievement of science that there is monitoring to that degree.
So, if you had stated the units properly, you could have avoided all that stuff.
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Hi all,
BC
Which units are not correct ?
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Hi all,
BC
Which units are not correct ?
Length of day variations/fluctuations in milli seconds as published today for the relevant dates. ( - = minus )
15/08/2021 - 0.4058
16/08/2021 - 0.6633
17/08/2021 - 0.9998
18/08/2021 - 1.0715
You should have said "milliseconds per year" or whatever.
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Hi all,
Ok thanks,
The numbers are how much from zero each day value is. Plus/minus
(in milli seconds)
Therefore my predictions/calculations were correct.
🌍🧐
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Hi all
Ok Halc
I have totally lost track of your point with this sidetrack. This new chart mostly illustrates the effects of external torque on Earth, but it does also suggest large scale sustained momentum transfer between air and ground.
What are you driving at with all these posts? Are you in denial of momentum conservation? Are you in denial that thermal energy from the sun can generate air currents on Earth? It is really unclear since the posts have no common thread and seem just haphazard.
Righto you now accept that there is a on going large scale transfer of momentum between the atmosphere and the solid Earth, measurable (daily) as changes in Earth rotation rate,
and polar motion
Yes, electromagnetic energy from the Sun generates wind in the Earths atmosphere
This from the go to guy;
The atmospheric angular momentum (AAM) is that due to the motion of the winds and to the changes in mass distribution
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
Now I am repeating myself at your request,
So current theory needs to state, where the initial action reaction pairing, due to the spontaneous convection occurs in the earths uncoupled atmosphere.
Given the motion/momentum generated, impacts things like sail boats mountains trees. and oceans, with varying degrees of partially inelastic collisions, that would alter the vector total expected from a closed system.
Therefore it needs to be demonstrated if the solid earth can/cannot discriminate between these collision and a meteorite, in regards to the potential change of momentum. that is created in the atmosphere, current theory requires the solid earth to be able to discriminate.
You ask where I am going with this, you need to consider the implications of what I am suggesting and the subsequent questions that raises.
gem
However this is only a preamble that links to several other phenomena,
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Righto you now accept that there is a on going large scale transfer of momentum between the atmosphere and the solid Earth, measurable (daily) as changes in Earth rotation rate,
and polar motion
On average, that transfer is zero in the long term.
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Hi all,
Ok BC
On average, that transfer is zero in the long term.
Appearances can be deceptive, there are patterns in the data that suggest that may not be the case.
Halc
You then have to stop mixing talk about the real Earth with its friction, mass transfer, momentum fluctuations and all, and work with an ideal case, say a frictionless solid ball with spinning atmosphere that is initially totally stationary relative to its rotating reference frame, and then heated on one side. That will generate air currents. The vertical reaction pair is with the surface which does not give with the altered pressure. The initial horizontal action-reaction pair would be atmosphere on either side of the heated region which would either move apart or together (probably the latter). Either way, you now have wind (air no longer stationary in that rotating frame), but no sailboat since our ideal planet is frictionless, so nothing for the keel to dig into. Can't even fly a kite.
If the planet is not frictionless as you describe, then yes, the atmosphere ceases to be a closed system, but planet+atmosphere still can be, and conservation laws still apply. Are you asserting otherwise, or is you point something else? It's still unclear.
OK I don't quite agree with the vertical action reaction statement but putting that to one side and concentrate on the sideways component, being equal and opposite as per Newton, we end up with wind, interacting with the real surface, with one eye on the friction-less surface scenario, do we not end up with the situation in reality, where this new constantly generated movement of mass relative to the surface, then requires a perfectly balanced coefficient of friction in all directions over the globe.
Otherwise the status quo of the friction-less solid sphere is not replicated, and treating the earth and the atmosphere as a closed system fails.
https://en.wikipedia.org/wiki/Anemometer
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Appearances can be deceptive, there are patterns in the data that suggest that may not be the case.
There's one pattern- tidal drag- though it's not entirely obvious if the air pulls the ground or pushes it. (Does the tide mainly affect the sea and that affects the solid earth which affects the air, or does the pull of the tide on the air have more effect?)
There are tiny effects due to radiation pressure and solar wind.
But the conservation of angular momentum means that the solid earth and the oceans and the atmosphere taken as a whole are an isolated system.
So, apart from the small external effects, the angular momentum of the system is fixed and, because it can't change shape or size much (Earthquakes and melting polar ice can change the shape slightly; weather changes the position of the water a bit), the period of rotation is nearly fixed.
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Hi all,
So BC,
Yes that is a very good description of the current understanding, treating the earth and atmosphere, as a combined closed system, as mainstream science absolutely does.
Halc
Those equal and opposite laws apply vertically just as much as horizontally. Air will not accelerate without a net force exerted on it by something else.
Yes but not initially involving the surface, as you stated, warm air is displaced by cooler/denser air
In regards to the point made with to many commas,,,,, then lets try the globe vessel suspended on a friction-less gimble
filled with water, with a fixed amount of angular momentum, lets set it at zero.
Lets then apply heat centrally, underneath it (as we are not in an inertial reference frame) so we get a central column of convection upwards and an equal flow downwards around the sides of the globe.
If the inside of the vessel surface coefficient of friction was designed to replicate an anemometer to favour more friction in one direction, what effect would this have on the momentum of the system ?
https://en.wikipedia.org/wiki/Anemometer
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Those equal and opposite laws apply vertically just as much as horizontally. Air will not accelerate without a net force exerted on it by something else.
Yes but not initially involving the surface, as you stated, warm air is displaced by cooler/denser air
You're mixing up cause and effect. This only happens after the air starts moving. Net motion cannot happen without the force exerted by an external object.
So initially, heat just changes the local pressure. In the absence of force from the ground, that air would expand equally in all six directions until a new equilibrium is found. This is why a balloon in orbit cannot get to a higher orbit just by heating it. So the ground resists the motion downward. Increased pressure at the ground pushes the warm air up. One it moves (up and out at first) it becomes less dense and then buoyancy takes over, but even that requires reaction force from the ground.
Fluid dynamics is incredibly complicated and requires considerable prerequisite calculus courses that I'm not going into. I'm trying to keep it simple.
In regards to the point made with to many commas,,,,, then lets try the globe vessel suspended on a friction-less gimble
We all decided that it wouldn't work. The system is subject to external torque. The (spherical) vessel will not necessarily rotate in the opposite direction of the rotation of its contents.
Lets then apply heat centrally, underneath it (as we are not in an inertial reference frame)
We're not? You said it was all stationary at first. Sounds pretty inertial except for the gravity.
so we get a central column of convection upwards and an equal flow downwards around the sides of the globe.
Yes. I had tried applying the heat to the sides to get rotation to attempt to get the vessel to rotate the other way to preserve angular momentum, but it wasn't preserved because we didn't keep the system closed.
If the inside of the vessel surface coefficient of friction was designed to replicate an anemometer to favour more friction in one direction
An anemometer has equal friction in all directions, else equal magnitude wind from different directions would make it rotate at different speeds, which defeats the purpose of a speed measuring device. But I think you mean put up sails on the sides of the vessel that inhibit flow in one direction more than another. So all this fluid is going down the sides and up the middle. If we have asymmetrical friction on the sides, we can get the vessel to rotate, but not necessarily the system.
what effect would this have on the momentum of the system ?
Linear momentum: So long as the center of gravity doesn't go anywhere, the system does not move. If the center of gravity goes up or down, external force from the gimbal will counter it, leaving it with zero momentum.
Angular momentum: So long as the center of gravity remains over the gimbal axes, the system angular momentum must remain zero. You're introducing asymmetries with the variable friction along the vessel (similar to the asymmetries of applying the heat off-center), so I suspect that the center of gravity will move to one side, in which case the system will be subjected to an external torque applied through the gimbal and will acquire nonzero angular velocity. The water will rotate and take the vessel with it since it has friction.
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what effect would this have on the momentum of the system ?
Have you applied an external force or an external torque to the system?
If not, then nothing will happen to the momentum or angular momentum of the system.
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Is it my imagination or does this thread have parallels with this one
https://www.thenakedscientists.com/forum/index.php?topic=80717.0
where one of our esteemed members forgot about the conservation laws?
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Hi all,
Ok, Halc
gem; my bold
Therefore changing the density which triggers convection due to buoyancy spontaneously creating upthrust, exerted as a result of the weight of the denser overlying fluid. "this can only occur in a non inertial frame of reference"
Halc
You can apply an inertial frame of reference on any system you like. Your assertion in quotes still suggests that convection currents cannot occur with say gas in a box in deep space, which is about as inertial as it gets. That assertion is dead wrong. You heat gas, it expands, and that expansion is motion (convection)
I don't believe you can apply an inertial reference frame if buoyancy is occurring, my statement of convection due to buoyancy is dead right, in your example of a box in deep space with energy added will not get convection due to buoyancy, unless you accelerate the box. (non inertial reference frame)
Halc
Not generated. Transferred. It has to come from somewhere, so an equal and opposite torque needs to be transferred to the ground or the air or somewhere. Despite the asymmetry, a continuously spinning anemometer in steady wind has no net torque on it. The net transfer is only needed to speed it up or slow it down.
I believe the torque is generated from the frictional forces of the velocity of the mass of air colliding with the cups and circular design of the apparatus.
A constant torque force is applied/transferred to the earths surface when a continuously spinning anemometer in steady wind is anchored to it, given the frictional force that is occurring.
Therefore it still needs to be demonstrated the solid earth can discriminate between these collision and a meteorite, in regards to the potential change of momentum of the solid earth. from the dynamical motion that is created in the atmosphere,independently of the solid earth through buoyancy/convection expansion and contraction, pressure differences and subsequent collisions/transfers between the atmosphere and the surface, .
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I believe the torque is generated from the frictional forces of the velocity of the mass of air colliding with the cups and circular design of the apparatus.
A counter-torque is generated on the air. - A straightforward application of Newton's 3rd law.
That, in turn exerts a torque on the solid Earth.
But the angular momentum of the whole ( rock, air, anemometer, bloke holding it ) system remains unchanged.
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Hi all,
BC
But the angular momentum of the whole ( rock, air, anemometer, bloke holding it ) system remains unchanged.
Halc
Buoyancy is not a violation of momentum conservation. It cannot generate new momentum, so to speak. The total remains constant for a closed system.
When the forces are not balanced, due to expansion of the atmosphere that's heated, the volume/droplet of air accelerates, due to the upthrust of the buoyancy effect.
This changes momentum due to the acceleration of bulk motion of fluids, the rate of change in momentum is equal to
Δmomentum = ρ.Δv/Δt
This is equal to the net force on the volume/droplet.
these forces that change the momentum of a volume/droplet of a fluid come from the gradient of the pressure and gravity.
This occurs due to energy input from outside the system under consideration doing work on the system to increase the volume occupied by mass, which then facilitates the separate spontaneous buoyancy/convection action/reaction.
Which happens independently of the solid surface, that it later interacts with via partially inelastic collisions/friction
Therefore the conditions of conservation of momentum are not met.
Halc
I don't know what you mean by 'discriminate'. It is an epistemological word
If you don't like the use of the word discriminate, I am happy to switch it to differentiate :D
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When the forces are not balanced, due to expansion of the atmosphere that's heated, the volume/droplet of air accelerates, due to the upthrust of the buoyancy effect.
This changes momentum due to the acceleration of bulk motion of fluids, the rate of change in momentum is equal to
Δmomentum = ρ.Δv/Δt
This is equal to the net force on the volume/droplet.
these forces that change the momentum of a volume/droplet of a fluid come from the gradient of the pressure and gravity.
This occurs due to energy input from outside the system under consideration doing work on the system to increase the volume occupied by mass, which then facilitates the separate spontaneous buoyancy/convection action/reaction.
All very nice up to here, and doesn't disagree with what we've been saying. Some details are missing like rotation that is inevitable with movement in a rotating frame of reference.
Which happens independently of the solid surface
This hasn't been demonstrated. For one, the solid surface provides the reaction force to the gravity, required for the buoyancy, so it is hardly independent of the surface. But as I said, a ball of gravitationally bound gas would still have buoyancy of low density regions, so the surface being a solid isn't a requirement, but the forces from below cannot be ignored.
Therefore the conditions of conservation of momentum are not met.
This also has not been demonstrated, and is an assertion that is not even remotely plausible. Are you saying the warm mass can accelerate upward with no equal and opposite reaction change of momentum elsewhere?
At least we finally have a hard assertion of violation of the conservation laws. You've been hesitant to just come out and say it.
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This changes momentum due to the acceleration of bulk motion of fluids, the rate of change in momentum is equal to...
Zero.
It's a conserved property; it can't change, so the rate of change is zero.
If you get a different answer then you are not looking at the whole picture.
Essentially, you consider the hot air rising, but you ignore the cold air falling.
The changes in momenta cancel out.
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This hasn't been demonstrated.
The presence of convection currents in the Sun shows that you don't need a solid surface.
It is sufficient that you have other matter to "push against".
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Therefore the conditions of conservation of momentum are not met.
The sole condition for that (provided that you look at the whole system) is that the laws of physics are symmetrical with regard to a change in position.
And that condition is met, universally.
https://en.wikipedia.org/wiki/Noether%27s_theorem
Consider , for the minute, the Earth and Sun (and the moon if you like) as the "system"
If you are saying that the Earth experiences a torque due to sunshine then the Sun must experience a counter-torque. Otherwise the angular momentum of that whole system is not conserved, and that suggestion is forbidden.
Since the torque provided to the Earth depends on the weather, the counter-torque must also do so.
What is the mechanism by which the Sun "knows" what the weather is like on Earth so that it can adjust its angular momentum accordingly?
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Therefore it needs to be demonstrated if the solid earth can/cannot discriminate between these collision and a meteorite, i
The Earth doesn't make the distinction, but we do.
We distinguish between "the atmosphere", which is part of the Earth and "the meteor" which is not part of the Earth.
There are more interesting cases; when we launched a rocket to take people to space, we slightly altered the rotation of the rock we are stood on.
But when that rocket landed, we re-established the length of the day (give or take the loss of momentum carried by the exhaust fumes).
However, if you consider the system which includes the rocket, the earth and all those fumes, the angular momentum (and linear momentum) remained unchanged through the whole process..
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Hi all,
Ok quite a lot to cover so may do it over a couple of posts.
Halc my bold
At least we finally have a hard assertion of violation of the conservation laws. You've been hesitant to just come out and say it.
I don't believe that's what I stated,
gem
Therefore the conditions of conservation of momentum are not met.
So moving on to
gem
Which happens independently of the solid surface
Halc
This hasn't been demonstrated. For one, the solid surface provides the reaction force to the gravity, required for the buoyancy, so it is hardly independent of the surface.
BC
The presence of convection currents in the Sun shows that you don't need a solid surface.
It is sufficient that you have other matter to "push against".
That's how I understand it.
Now BC I don't believe we can apply the argument for Noether's theorem in defense of the dynamics under consideration, but wouldn't like to comment against it in regards to the 3 body system you set out for the Sun Earth and Moon.
below a direct quote from the link you provided;
"Noether's first theorem states that every differentiable symmetry of the action of a physical system with conservative forces has a corresponding conservation law"
If you follow the link for the "action" you get;
Action has dimensions of energy × time or momentum × length, and its SI unit is joule-second
and if you follow conservation law you get;
in physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time.
given whats under discussion is down to an input of energy to the system, so it doesn't tick that box
and the "momentum x length" value is altered for a point mass during these dynamics it doesn't tick that box either.
BC you have raised two other points and Halc there's also something you've raised, that I will try to respond to tomorrow
:)
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I don't believe that's what I stated,
Therefore the conditions of conservation of momentum are not met.
OK, I perhaps see what you meant. The conditions are met though. The closed system has had no external torque/force applied to it, so its momentum is unchanged by the accelerating mass within. The accelerating low-density mass of air is not a closed system of course, just part of one.
So moving on to
Which happens independently of the solid surface
The presence of convection currents in the Sun shows that you don't need a solid surface.
It is sufficient that you have other matter to "push against".
That's how I understand it.
And I agree. I've said that buoyancy doesn't require a solid or liquid to push against. But the ground is there in this case, and the atmosphere is definitely getting force applied by it. It seems quite inaccurate to assert that it plays no role in the acceleration of the mass of low density air.
It sounds like you're still working things out.
Consider some complications. We start with the warming of a large region and all the lower atmosphere becomes correspondingly less dense. It can't easily go up because it all wants to go up and all the cold air above wants to go down and they're effectively in each other's way. It's an unstable equilibrium of sorts eventually resulting in turbulence and chaotic motion. That that chaotic motion quickly organizes itself into more stable patterns that we see.
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I don't believe that's what I stated,
It is what you stated.
Therefore the conditions of conservation of momentum are not met.
It may not be what you meant, but it is what you said.
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given whats under discussion is down to an input of energy to the system, so it doesn't tick that box
True.
But momentum (apart from radiation pressure) is not added to the system, so it closed in that regard.
You need to recognise that both quantities are conserved and you can't magically change one into the other.
BC you have raised two other points and Halc there's also something you've raised, that I will try to respond to tomorrow
Take your time.
The Moon, Sun and Earth aren't going anywhere.
I'm sure we look forward to your explanation of Earth's weather influencing the Sun's rotation.
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and if you follow conservation law you get;
in physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time.
given whats under discussion is down to an input of energy to the system, so it doesn't tick that box
I already explained this to you in a previous thread.
It is, as you keep stressing the importance of the fact, an isolated system.
The atmosphere can't push the world for the same reason that the man on the truck bed can't push the truck.
You seem to be trying to elevate the terms isolated and closed to the status of holy writ.
It's not magical.
Classically:
(1) A system to which you can't add or remove stuff will not change mass
(2) A system to which you can't add or remove energy will change energy
(3)A system to which you can not apply a force will not change momentum.
(4) A system to which you can't apply a torque will not change angular momentum.
Those are pretty much tautologically true.
Since Einstein's day the first pair of those are a bit more flexible, but only in a very clearly defined way.
Historically, in thermodynamics it was important to lump together some of those statements of the obvious.
If you have a system to which you can add or subtract matter then you can't sensibly define what it will do- because it depends on the matter you might add.The same goes for applying forces to it. Those forces just complicate the issue.
So, they invented the "closed system" where tautologies 1 and 3 apply.
Similarly, for some calculations, you want to prevent energy entering or leaving the system.
And that's why they invented the "isolated system"- as a shorthand for a system where tautology 2 also applies.
If thermodynamics dealt with angular momentum they would have invented another term for systems where tautology 4 also applied. They didn't.
But, if they had done, the Earth would be on the list.
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Hi all,
Ok Halc,
lets try and address the solid surface issue,
first the atmosphere has weight and does have an interface with the surface be it on land or sea, so contributes the normal force perpendicular to the surface, that stops the atmosphere being in free-fall.
However as stated previously ;
"When the forces are not balanced, due to expansion of the atmosphere that's heated, the volume/droplet of air accelerates, due to the upthrust of the buoyancy effect."
There is however an aspect that the surface is known to participate in the change in motion of the atmosphere/solid earth and due to the change in atmospheric pressure rather than the buoyancy/convection.
Specifically, the mechanism causing this are normal torques against topography/frictional coefficient by a varying atmospheric pressure.
Therefore leaving directional coefficient of friction in this regard being a major factor.
Similarly to the value of tangential torques from the lowest level of the winds upon the Earth’s surface being influenced by directional coefficient of friction.
BC
The Earth doesn't make the distinction, but we do.
We distinguish between "the atmosphere", which is part of the Earth and "the meteor" which is not part of the Earth.
Yes we do but I have not seen yet how the physical aspect of the tangential collision of a meteor with the surface is different, if the increase in motion of an air molecule is achieved without a increased force on the earths surface, and subsequent collision with it.
Some times it requires someone to suggest a different direction.
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
The earliest observations of changes in length of day were made at Paris Observatory by Stoyko
and Stoyko (1936), who observed the annual variation of length of day. In 1948, Victor Starr of the
Massachusetts Institute of Technology (MIT) noted that the atmosphere need not conserve angular
momentum, and could share it with the Earth below. Starr started the General Circulation Project
at MIT, and one of its features was calculations of fluxes and changes in atmospheric angular
momentum.
BC
Take your time.
The Moon, Sun and Earth aren't going anywhere.
I'm sure we look forward to your explanation of Earth's weather influencing the Sun's rotation.
Given the pressure and heat differences and the dynamics that are generated by the EMR input, there is no requirement for the earths weather to impact the Suns rotation.
However I like your logic as there are a few consequence's to what I am postulating that would seem to be in a similar vein, you've just not cottoned on to them yet.
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I have not seen yet how the physical aspect...
Well, if you keep thinking about it, and the conservation laws, you might work it out in the end.
Given the pressure and heat differences and the dynamics that are generated by the EMR input, there is no requirement for the earths weather to impact the Suns rotation.
There is if you want the Sun to provide angular momentum to the Earth.
If A pushes B then B must push back on A- or have you decided to add Newton's laws to the ones you are already ignoring?
However I like your logic as there are a few consequence's to what I am postulating that
The consequence of my logic is that your idea is flat out wrong.
you've just not cottoned on to them yet.
The evidence so far shows that you are not clever enough to patronise me. Don't bother trying.
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Victor Starr of the
Massachusetts Institute of Technology (MIT) noted that the atmosphere need not conserve angular
momentum, and could share it with the Earth below.
Did you think anyone had disputed that fact?
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Hi all,
BC
Did you think anyone had disputed that fact?
Given it was down to a necessity to explain the newly discovered three millisecond annual variation in LOD I believe it was the explanation of its time, but don't doubt it wont have been unanimously accepted.
https://en.wikipedia.org/wiki/Day_length_fluctuations#/media/File:Deviation_of_day_length_from_SI_day.svg
The annual component of the change of the length of day of approx 0.34 ms corresponds then to a increase in wind speed 0.9 m/s of the whole of earths atmosphere, maximizing like clockwork every year on/around February 3 each year.
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OK, did you think anyone here had disputed it?
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The annual component of the change of the length of day of approx 0.34 ms corresponds then to a increase in wind speed 0.9 m/s of the whole of earths atmosphere, maximizing like clockwork every year on/around February 3 each year.
And, if you look at the next bit of the wiki page you see
"10‑day fluctuations of the order of 0.1 milliseconds."
And, if you are right, if the planet + atmosphere are slowing and speeding due to a torque from the Sun then they (by Newton's 3rd law) must be exerting a torque on the Sun.
What's the mechanism for that?
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As ice and snow melt and the water runs into the sea, so the moment of inertia of the planet decreases, so it spins faster. No surprise there.
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Hi all,
Ok lots of points some I will try to address others will be irrelevant, going forward or come into focus along side new questions,
Halc and BC
Halc
The blob and solid surface issue I responded to yesterday, on the importance of topography/coefficient of friction.
BC I understand the thrust of your request, for the mechanism, but point you back to my previous response and the mechanism/dynamics that are occurring.
In 1948 Starr postulated the aspect of transfer of momentum between the atmosphere and solid Earth, In 1970 he postulated this about Earths atmosphere:
"In most fluid systems differences in velocity are obliterated by positive viscosity. In some rotating systems, however, non uniform flows can be maintained by negative viscous effects due to eddies. where the fluid motion is maintained by a constant input of energy."
Alan, regarding the ice and snow melting wouldn't we have to look at the angular velocity of where its stored to where it ends up ? , there is potential for a paradox given we are now experiencing some of the shortest days, since timings began with atomic clocks, and reduction in viscosity cannot fit into current theory.
BC
OK, did you think anyone here had disputed it?
Yes
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BC I understand the thrust of your request, for the mechanism, but point you back to my previous response
If that response had been adequate, I wouldn't have asked.
What's the mechanism by which the Earth pushes back on the Sun in order to conserve angular momentum for the (isolated) system as a whole??
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Quote
OK, did you think anyone here had disputed it?
Yes
Please show us where anyone here said that the solid Earth and its atmosphere don't exchange angular momentum?
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Hi all,
BC my bold
What's the mechanism by which the Earth pushes back on the Sun in order to conserve angular momentum for the (isolated) system as a whole??
So at the risk of repeating myself the Earth is not an isolated system and you cannot ignore the fact that the action/interaction of the surface and atmosphere has the SI unit of joule-second, for momentum.
Also these transfer happen by way of friction and friction is a non-conservative force.
BC
Please show us where anyone here said that the solid Earth and its atmosphere don't exchange angular momentum?
I have been stating on numerous posts, that the transfers between the solid surface and the atmosphere are partially inelastic, So the exchange of angular momentum will not be able to return the same value of momentum back to the rotating solid earth, to maintain the LOD.
The rotation of the Earth is actually its "terminal angular velocity"
IE a balance between frictional couplings and torque/accelerating forces.
Similar to sky divers who can alter their terminal velocity. :)
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Also these transfer happen by way of friction and friction is a non-conservative force.
At the atomic level, all the interactions are conservative.
So, in every interaction momentum and energy are conserved.
When you add them all up the total is also conserved.
Momentum is (unlike energy) conserved in non-elastic collisions where heat is formed.
So at the risk of repeating myself the Earth is not an isolated system
The problem is not the repletion; the problem is repeating an error.
I already explained this to you in a previous thread.
Quote from: Bored chemist on 10/09/2020 08:47:48
It is, as you keep stressing the importance of the fact, an isolated system.
The atmosphere can't push the world for the same reason that the man on the truck bed can't push the truck.
You seem to be trying to elevate the terms isolated and closed to the status of holy writ.
It's not magical.
Classically:
(1) A system to which you can't add or remove stuff will not change mass
(2) A system to which you can't add or remove energy will change energy
(3)A system to which you can not apply a force will not change momentum.
(4) A system to which you can't apply a torque will not change angular momentum.
Those are pretty much tautologically true.
Since Einstein's day the first pair of those are a bit more flexible, but only in a very clearly defined way.
Historically, in thermodynamics it was important to lump together some of those statements of the obvious.
If you have a system to which you can add or subtract matter then you can't sensibly define what it will do- because it depends on the matter you might add.The same goes for applying forces to it. Those forces just complicate the issue.
So, they invented the "closed system" where tautologies 1 and 3 apply.
Similarly, for some calculations, you want to prevent energy entering or leaving the system.
And that's why they invented the "isolated system"- as a shorthand for a system where tautology 2 also applies.
If thermodynamics dealt with angular momentum they would have invented another term for systems where tautology 4 also applied. They didn't.
But, if they had done, the Earth would be on the list.
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I have been stating on numerous posts, that the transfers between the solid surface and the atmosphere are partially inelastic, So the exchange of angular momentum will not be able to return the same value of momentum back to the rotating solid earth, to maintain the LOD.
Momentum is conserved during inelastic collisions.
Energy is conserved in inelastic collisions.
Only kinetic energy is not conserved- some is converted to thermal energy.
So the elasticity of a collision is immaterial to a discussion on momentum.
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Hi all
For the elasticity of the collisions to be immaterial, then it requires explanations of phenomena such as the transfer of momentum via sound waves the majority that also end up as heat, retrieved to the angular momentum.
For your position to be credible.
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then it requires explanations of phenomena such as the transfer of momentum via sound waves t
The explanation isn't very complex.
Molecule A hits molecule B.
Momentum and energy are conserved in this process.
So they are also conserved in any process which is a succession of repetitions of that step.
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Hi all
ok
then it requires explanations of phenomena such as the transfer of momentum via sound waves t
The explanation isn't very complex.
Molecule A hits molecule B.
Momentum and energy are conserved in this process.
So they are also conserved in any process which is a succession of repetitions of that step.
Ok so the collision occurs transferring momentum away from direction of the angular velocity of the solid Earth to the atmosphere, and its a partially inelastic collision, in which direction do these subsequent conserved momentum of sound waves of molecule A and molecule B colliding go ? that enable the 100% transfer back in the correct direction later, actually go ?
Given the Inverse square law of sound waves how does that enable the 100% transfer of momentum back in the correct direction later.
http://hyperphysics.phy-astr.gsu.edu/hbase/Acoustic/invsqs.html
Also given the time aspect between transfers how do you exclude thermal radiation in your calculations spontaneously transfering away from that side of the equation, and away from the isolated system the joules in your original calculation ? which are the SI values of the action under consideration ?
https://en.wikipedia.org/wiki/Thermal_radiation
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Ok so the collision occurs transferring momentum away from direction of the angular velocity
You are muddling a whole bunch of things there.
Please clarify your question in terms of angular momentum;
linear momentum and energy, all of which are strictly conserved.
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Given the Inverse square law of sound waves
Are you aware that sound does not follow the inverse square law?
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Also given the time aspect between transfers how do you exclude thermal radiation in your calculations spontaneously transfering away from that side of the equation
I don't need to.
All transfers, including those carried by photons, conserve momenta and energy
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HI all,
OK BC
You are muddling a whole bunch of things there.
Please clarify your question in terms of angular momentum;
linear momentum and energy, all of which are strictly conserved
All of which are strictly related in wave form, for example,
momentum = energy/phase velocity.
wave momentum is related to wave energy in
a simple and seemingly universal way. there are many different
kinds of wave motion that links them, for example, electromagnetic waves, sound waves, water waves, and certain kinds of traveling waves such as one's on strings under tension.
BC
Are you aware that sound does not follow the inverse square law?
Sorry BC, I don't agree, and I did provide a link in the previous post that states:
The sound intensity from a point source of sound will obey the inverse square law
also given the sound intensity is defined as the sound power per unit area.
http://hyperphysics.phy-astr.gsu.edu/hbase/Acoustic/invsqs.html
gem
Also given the time aspect between transfers how do you exclude thermal radiation in your calculations spontaneously transferring away from that side of the equation
BC
I don't need to. All transfers, including those carried by photons, conserve momenta and energy
The problem with that is,
the momentum and energy of sound is carried off in all directions.
which brings us back to, how is the vector direction total maintained for later transfer back to the angular momentum of the solid earth, due to the partially inelastic collisions sending momentum away from the transfer point, in all directions ?
indeed as you say photons conserve momenta and energy but the system does not.
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Sorry BC, I don't agree,
It is unfortunate that you do not agree with reality, but it is no surprise to me.
"Stokes's law of sound attenuation is a formula for the attenuation of sound in a Newtonian fluid, such as water or air, due to the fluid's viscosity. It states that the amplitude of a plane wave decreases exponentially with distance travelled,"
https://en.wikipedia.org/wiki/Stokes%27s_law_of_sound_attenuation
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The problem with that is,
the momentum and energy of sound is carried off in all directions.
Which means that, on average, it is zero.
So, as it fades, there's no problem.
which brings us back to, how is the vector direction total maintained
It is zero.
As you say, it points equally in all directions and the only way to do that is to have all the vectors "cancel out".
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indeed as you say photons conserve momenta and energy but the system does not.
Does it worry you that you are the only one who believes this?
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Hi all
OK BC
"Stokes's law of sound attenuation is a formula for the attenuation of sound in a Newtonian fluid, such as water or air, due to the fluid's viscosity. It states that the amplitude of a plane wave decreases exponentially with distance travelled,"
SO Acoustic attenuation is a measure of the energy loss of sound propagation in media. Most media have viscosity, and are therefore not ideal media. When sound propagates in such media, there is always thermal conversion of wave energy caused by viscosity.
Which just confirms what I posted earlier
gem
For the elasticity of the collisions to be immaterial, then it requires explanations of phenomena such as the transfer of momentum via sound waves the majority that also end up as heat, retrieved to the angular momentum.
Attenuation does not include the decrease in intensity due to inverse-square law geometric spreading. Therefore, calculation of the total change in intensity involves the inverse-square law and an estimation of attenuation over the path due to the viscosity of the medium.
gem
how is the vector direction total maintained for later transfer back to the angular momentum of the solid earth, due to the partially inelastic collisions sending momentum away from the transfer point, in all directions ?
BC
It is zero.
As you say, it points equally in all directions and the only way to do that is to have all the vectors "cancel out".
precisely the solid earth transferred angular momentum to the atmosphere reducing the solid earths momentum total.
Which as you say the momentum carried away by sound to be available to be transferred back later is ZERO
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Therefore, calculation of the total change in intensity involves the inverse-square law and an estimation of attenuation over the path due to the viscosity of the medium.
Well done.
You have caught up with what I said a while ago.
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precisely the solid earth transferred angular momentum to the atmosphere reducing the solid earths momentum total.
The solid earth (over a reasonable period of time) transfers ZERO momentum to the atmosphere.
Because the momentum carried by the sound sums to zero.
So the loss by the solid earth is zero.
If the Earth lost angular momentum to the air then the air would have to rotate WRT the earth.
Friction would slow it down again until the angular momentum was transferred back.
In the short term, there can be transfers (though they are very near zero on average across the Earth).
Averaged over time they are exactly zero. |(Give or take atmospheric tidal drag).
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Hi all,
OK
precisely the solid earth transferred angular momentum to the atmosphere reducing the solid earths momentum total.
Which as you say the momentum carried away by sound to be available to be transferred back later is ZERO
The solid earth (over a reasonable period of time) transfers ZERO momentum to the atmosphere.
Because the momentum carried by the sound sums to zero.
So the loss by the solid earth is zero.
If the Earth lost angular momentum to the air then the air would have to rotate WRT the earth.
Friction would slow it down again until the angular momentum was transferred back.
In the short term, there can be transfers (though they are very near zero on average across the Earth).
Averaged over time they are exactly zero. |(Give or take atmospheric tidal drag).
In the short term the transfers may seem small when we are considering fractions of a milli second daily variations in the
LOD "however they are not"
If we consider the approximate 0.3 of a milli second variation for one day given in the dates earlier, corresponds to a change in rotational kinetic energy of the solid earth in the region of 1.6 x 10^21 joules for the day in question.
These daily fluctuations are values that are not remotely near to zero.
The fact that the LOD appears consistent over longer time periods can also be explained
by a balance of frictional forces and accelerating forces ( terminal angular rotation) rather than the "born that way" explanation.
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If we consider the approximate 0.3 of a milli second variation for one day given in the dates earlier, corresponds to a change in rotational kinetic energy of the solid earth in the region of 1.6 x 10^21 joules for the day in question.
I realise it's terribly "English" to always talk about the weather but...
We are talking about the momentum transfers which definitely sum to zero over the long term.
You are talking about the transfer of heat from the earth to the air.
And that's broadly what drives the weather.
So it is irrelevant.
The hint is in the units.
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The annual component of the change of the length of day of approx 0.34 ms...
If we consider the approximate 0.3 of a milli second variation for one day
I don't know where the variation of 0.3ms in one day came from. This quotes an annual change, not a 1 day change.
The current long-term variation in Earth's rate of rotation is quoted as"+1.7 ms/d/cy", ie every century, it changes by 1.7 ms/day.
- So in a year, you could expect it to change by about 170μs/day (plus short-term variations which might take it up to 300μs/day over the course of some years).
See: https://en.wikipedia.org/wiki/%CE%94T_(timekeeping)#Earth's_rate_of_rotation
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Hi all,
If we consider the approximate 0.3 of a milli second variation for one day given in the dates earlier, corresponds to a change in rotational kinetic energy of the solid earth in the region of 1.6 x 10^21 joules for the day in question.
I realise it's terribly "English" to always talk about the weather but...
We are talking about the momentum transfers which definitely sum to zero over the long term.
You are talking about the transfer of heat from the earth to the air.
And that's broadly what drives the weather.
So it is irrelevant.
The hint is in the units.
MMMMM, I believe we already covered the universal link (momentum = energy/phase velocity.)
and if you look at the original question, the units for energy are correct and relevant, to put a scale to what you tried to dismiss as close to zero.
However if you prefer a 0.3 milli sec daily variation in LOD should give a corresponding ±Δ angular momentum
of aprox = 2.019 x 10^25 Kg m^2 for the day in question
Evan_au
I don't know where the variation of 0.3ms in one day came from. This quotes an annual change, not a 1 day change.
You probably need to look back to post 60 and a few on from there.
But to help, the LOD tends to skip about quite a bit more than you believe, also it has an annual speeding up in the northern hemisphere's summer and slowing down in its winter to a well established pattern of ± 1.5 milli sec as per the link below.
https://en.wikipedia.org/wiki/Day_length_fluctuations#/media/File:Deviation_of_day_length_from_SI_day.svg
And bringing that data into closer detail
https://datacenter.iers.org/singlePlot.php?plotname=FinalsDailyIAU1980-LOD-BULA&id=12
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In every single collision between any two atoms or between atoms and photons or phonons, momenta and energy are conserved.
So they must be conserved in every sequence and combination of collisions.
So they are conserved in total.
Get back to us when you can explain how arithmetic fails.
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believe we already covered the universal link (momentum = energy/phase velocity.)
Yes we did.
I pointed out that sound waves spread in all directions, so their vector sum is zero.
That cover it nicely.
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Hi all
BC
In every single collision between any two atoms or between atoms and photons or phonons, momenta and energy are conserved.
So they must be conserved in every sequence and combination of collisions.
How does the addition work for a non isolated system ?
https://en.wikipedia.org/wiki/Thermal_radiation
BC
I pointed out that sound waves spread in all directions, so their vector sum is zero.
That cover it nicely.
I think if you look back that was me.
gem
The problem with that is,
the momentum and energy of sound is carried off in all directions.
which brings us back to, how is the vector direction total maintained for later transfer back to the angular momentum of the solid earth, due to the partially inelastic collisions sending momentum away from the transfer point, in all directions ?
So the problem still remains, a daily change in angular momentum vector due to friction ending up as momentum with a positive value but no longer having a net direction to transfer back to maintain/conserve the LOD
Indeed the random motion of particles can decrease/increase depending on the balance of incoming/outgoing flux of radiation to the Non isolated system under consideration.
Also given the Non inertial conditions of the system this generates forces that can and does accelerate/change the momentum of a droplet of a fluid by altering the gradient of the pressure due to the dynamics of changing density in a gravity field due to the incoming/outgoing flux of radiation to the Non isolated non inertial system under consideration.
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How does the addition work for a non isolated system ?
If no particle leaves then no momentum leaves.
If the loss is (on average) isotropic then the net change in momentum is (on average) zero
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So the problem still remains, a daily change in angular momentum vector due to friction ending up as momentum with a positive value but no longer having a net direction to transfer back to maintain/conserve the LOD
As Paul Simon said, "the problem is all inside your head"
"The answer is easy , if you take it logically."
This "a daily change in angular momentum vector due to friction "
Just isn't real.
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I realise it's terribly "English" to always talk about the weather but...
For those who didn't get the point.
Gem talks of
1.6 x 10^21 joules for the day
which is quite a lot of energy
But a single hurricane can carry 10^20 J
So, if you consider the weather over the whole Earth, 1.6*10^21 J is not much
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Hi all,
BC
If the loss is (on average) isotropic then the net change in momentum is (on average) zero
The problem with that statement is the frictional coupling between the solid earth and its atmosphere converts anisotropic motion to isotropic motion.
Also as stated previously, the the Non inertial conditions of the system that generates forces that can and does accelerate/change the momentum of a droplet of a fluid by altering the gradient of the pressure due to the dynamics of changing density in a gravity field due to the incoming/outgoing flux of radiation to the Non isolated non inertial system under consideration is an example of anisotropy due to bulk flows of fluid with a background effect due to density, therefore favoring a specific direction.
So the problem still remains, a daily change in angular momentum vector due to friction ending up as momentum with a positive value but no longer having a net direction to transfer back to maintain/conserve the LOD
Moving on to the points regarding following the energy of the interactions I am highlighting here its the patterns of the fluctuations that could give a insight to the dynamics.
So looking at the annual 3 milli second variation in LOD ( Δ = 1.4477 x 10^22 Joules/day )
Given it was down to a necessity to explain the newly discovered three millisecond annual variation in LOD I believe it was the explanation of its time, but I believe there is alternative explanations that will be more satisfactory.
https://syrte.obspm.fr/astro/journees2019/FILES/salstein.pdf
The earliest observations of changes in length of day were made at Paris Observatory by Stoyko
and Stoyko (1936), who observed the annual variation of length of day. In 1948, Victor Starr of the
Massachusetts Institute of Technology (MIT) noted that the atmosphere need not conserve angular
momentum, and could share it with the Earth below. Starr started the General Circulation Project
at MIT, and one of its features was calculations of fluxes and changes in atmospheric angular
momentum.
BC
As Paul Simon said, "the problem is all inside your head"
"The answer is easy , if you take it logically."
This "a daily change in angular momentum vector due to friction "
Just isn't real.
;D
Its funny you should take the lyrics route to make a point, because I intend to do similar to start the explanation as to the real dynamics occurring.
Daft Punk Lyrics:
"Like the legend of the phoenix
Our ends were beginnings
What keeps the planet spinning
The force from the beginning"
So look out for "Are you a Daft Punk or a Newton of drift theory ?"
https://earthobservatory.nasa.gov/features/Wegener/wegener.php
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The problem with that statement is the frictional coupling between the solid earth and its atmosphere converts anisotropic motion to isotropic motion.
That bit of the system isn't where things are lost from, so momentum cannot be carried away into space.
You also seem not to recognise that, on any given day, the angular momentum of the atmosphere may be a bit higher or lower than the average but, the total angular momentum of the rock and the air is conserved.
It is conserved because there is no torque acting on the system (apart from a few thing we have discussed, such as the tides).
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Hi all,
BC
You also seem not to recognise that, on any given day, the angular momentum of the atmosphere may be a bit higher or lower than the average
So to be clear, I believe it is possible the average angular momentum total fluctuates, for the reasons given previously,
the sharing of angular momentum between the atmosphere and the solid earth became the go to explanation of the underlying annual fluctuation.
I believe it can be explained as terminal velocity.
BC
It is conserved because there is no torque acting on the system (apart from a few thing we have discussed, such as the tides).
That indeed is the current thinking, which I believe falls short of the physical reality, I think there is potential alternative explanations as to the fluctuations of LOD that indeed require forces not yet identified.
To explain these forces although a simple mechanism, I will have to go across a broad range of examples and correlations of physical events.
For example a potential link to galaxy rotation curves and G
:)
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I believe it can be explained as terminal velocity.
You can believe what you like but, since this is a science page, you will be ignored if you don't provide evidence.
To explain these forces although a simple mechanism, I will have to go across a broad range of examples and correlations of physical events.
If you are not talking about sailing ships, you should probably open a new thread.
Also, since you are putting forward a "new theory", your new thread shouldn't be in this section.