[gem (OP)]

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.
 

[Bored chemist]

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...)

[alancalverd]

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.     

[Bored chemist]

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.

[evan_au]

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

[alancalverd]

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.

[gem (OP)]

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 ?

[alancalverd]

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.

[Bored chemist]

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.

[alancalverd]

Every point of the planet is illuminated by the sun for exactly half of the year.

[Bored chemist]

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.

[gem (OP)]

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 ?

[Halc]

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.

[alancalverd]

That's an idea you can stick where the sun doesn't shine.

On the dark side of Venus?

[alancalverd]

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!

[evan_au]

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?

[gem (OP)]

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.

[Eternal Student]

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.   
- - - - - - -
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.

[Halc]

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

[Colin2B]

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.