[hamdani yusuf]

A 1 kg ball is attached by a 1 meter string to a 2 kg ball. The system is rotating at 1 rps in space far away from any other mass. At t1 the string breaks.
Is there any change in angular momentum?
Is there any change in linear momentum?

[Bored chemist]

A 1 kg ball is attached by a 1 meter string to a 2 kg ball. The system is rotating at 1 rps in space far away from any other mass. At t1 the string breaks.
Is there any change in angular momentum?
Is there any change in linear momentum?

No and No.
Because no torque acts on it there is no change in angular momentum
Because no force acts on it, there is no change in linear momentum.

[gem]

Hi all.
So in response to my statement of;

Also is the momentum carried Out in all directions by the sound waves included in your calculations?

Halc states:

Sound waves carry energy but not momentum. That means a perfectly elastic collision between objects cannot make any sound.  The inelastic collision can, but if the system is moving through a medium which carries sound (air), they're probably losing momentum due to friction with the air. That would constitute an external force upon the system.

This is not the case with Earth as a system.  It makes no sound as it travels through space.

To which I respond

That is incorrect, for your and subsequent readers information it can be calculated using
Momentum = energy/phase velocity

To which BC states

As Halc pointed out, the momentum carried by them is zero.
The symmetry argument has the advantage of being quick and obvious, which is why I used it.
Halc clarified that, a wave carries energy away from the system
In an elastic collision, no energy is lost so there can be no wave.
A wave that does not exist can not carry momentum

For some reason you have brought it up again.

So what Halc stated is clearly wrong as he believes sound waves don't  transfer momentum, and what BC states is disingenuous and incorrect given the dynamics under discussion are the frictional coupling of the atmosphere and the surface of the earth which are not perfectly elastic coupling therefore sound waves do exist and are transferring momentum out symmetrically, which as stated previously under the laws of conservation, a percentage of said momentum came from the angular momentum of the solid earth/atmosphere total.

So as I stated before, for you position to be credible you need to address this continuous transfer away from the angular momentum budget.

Now in regards to my example of a direct conversion of linear momentum to angular momentum using pistons and crankshafts and flywheel in response to BC statement of;

  “So, for example, you can not convert linear momentum into angular momentum”

Neither BC or Halc thought it was a correct example and wanted to take the engine apart, or move the boundaries of the system

So you have made clear what you consider the boundaries in earlier posts so try this heat engine for an example of linear to rotational/angular momentum.

https://www.cnbc.com/2016/10/07/heres-why-hurricanes-spin-counterclockwise-in-the-north.html
https://en.wikipedia.org/wiki/Coriolis_force

[Bored chemist]

And still, at every stage where a molecule hits a molecule, momentum is conserved so the momentum overall is conserved.

So you still can't convert a scalar (like energy)  to a vector (like momentum) this way.
You still can't convert linear to angular  momentum and you still can't convert angular to linear.
So the Earth's angular momentum is conserved and the earth just keeps turning.
As Stonehenge proves it has been doing for millennia.

Which of those don't you understand?

So you have made clear what you consider the boundaries in earlier posts so try this heat engine for an example of linear to rotational/angular momentum.

The coriolis force happens because momenta are conserved.

[nathmaheshmp]

Ignoring second-order effects such as thermal winds, the source of kinetic energy is the rotational energy of the planet, so it must eventually stop spinning.

[Bored chemist]

Ignoring second-order effects such as thermal winds, the source of kinetic energy is the rotational energy of the planet, so it must eventually stop spinning.

That is a second (third?) order effect.
The first order is that it just keeps turning.

[gem]

Hi all
So given the dynamics under discussion are not dissimilar,
 

BC you state

And still, at every stage where a molecule hits a molecule, momentum is conserved so the momentum overall is conserved.

The key word there is "overall",

which still leaves outstanding an explanation or comment as to this aspect;

the frictional coupling of the atmosphere and the surface of the earth which are not perfectly elastic coupling therefore sound waves do exist and are transferring momentum out symmetrically, which as stated previously under the laws of conservation, a percentage of said momentum came from the angular momentum of the solid earth/atmosphere total.

So as I stated before, for you position to be credible you need to address this continuous transfer away from the angular momentum budget.

Also BC when you state

The coriolis force happens because momenta are conserved.

yes agreed but it starts out as linear momentum and ends up with rotation/angular momentum
https://en.wikipedia.org/wiki/Coriolis_force#/media/File:Coriolis_effect14.png

Also in ideal gas laws one of the freedoms of movement of a gas molecule is rotation and if a molecule with zero spin has a linear collision "off center" spin/rotation is a consequence.

Now your stonehenge analogy is a most relevant point and similarly will require addressing.

BC just seen your most recent post stating the first order is it just keeps turning, which is quite relevant to the stonehenge comment i just wrote.

So keeps turning yes but not as consistently/smoothly as you might think, for example

28 th sept LOD was - 0.5142 milli sec
29 th sept LOD was - 0.3812 milli sec

 which is a pretty standard daily fluctuation, which represents Δ of approximately 6.55 x 10^20 Joules of kinetic energy

[Bored chemist]

yes agreed but it starts out as linear momentum and ends up with rotation/angular momentum

there is, absolutely, no stage during the whole process where the total angular or linear momentum -measured in any sensible way- will actually change.

And the same is true of the Earth.

which still leaves outstanding an explanation or comment as to this aspect;

No, it does not.
because the momenta are conserved at every scale.

which is a pretty standard daily fluctuation, which represents Δ of approximately 6.55 x 10^20 Joules of kinetic energy

Congratulations on spotting something we all accepted ages ago.
There's a tidal drag from the Moon etc.
But that's a different effect from the ones you are talking about where the atmosphere slows the Earth down. (notably it is different because it is  real).

[Bored chemist]

which is a pretty standard daily fluctuation, which represents Δ of approximately 6.55 x 10^20 Joules of kinetic energy

Or not.
You can't say if that's a change in momentum or moment of inertia.
(Obviously, it's not a sudden change in momentum, because that's impossible- but you seem to have dispensed with logic)

[gem]

Hi all,
So BC you raise one good point

You can't say if that's a change in momentum or moment of inertia.

Just for clarity for those that may not understand your point below is quite a good demonstration.

I believe there are some quite sophisticated gps tech that monitors the solid earth's flexing so along with that data and length of day and weather patterns there is good reference points, However as mentioned earlier this is an area of active research, and there are obviously very difficult aspects to getting a handle on the dynamic atmospheres momentum total at any one time but there is noted discrepancies.
But it is currently postulated most of daily variations are credited to exchanges of momentum between the solid earth and the atmosphere. 

[Jaaanosik]

...
OK, I have a system with say linear momentum of 100 kg.m/sec in the x direction relative to inertial frame F.
I add 1 KJoule of energy to the system.  What is its momentum after I do that?
You can have anybody you like 'looking' at the system.

More specifically, how much kinetic energy does it have if I add 1 KJoule of heat to the system? That question actually can be answered.
...

Heat can generate rotational kinetic energy, angular momentum in a closed system (no mass in/out, just heat energy in/out).
It depends on the efficiency of the system how much rotational kinetic energy the system would have.

[Halc]

But it is currently postulated most of daily variations are credited to exchanges of momentum between the solid earth and the atmosphere.

Perhaps most, or perhaps not. I don't know. The Earth below is hardly a solid thing, and magma currents within must have an incredible inertial effect, compared to an atmosphere that masses only a millionth of the planetary mass.

My point has been that the total momentum cannot be affected by this at all, and such variations between one component of the system and another are temporary fluctuations and cannot be cumulative.  If the atmosphere loses momentum to the ground one day, it must gain it back another day, else eventually one would move at a significantly higher or lower angular rate than the other, and the friction would be imbalanced.

Real torque on the other has IS cumulative, which is why Earth has been slowing down for billions of years, relinquishing its spin momentum to orbital momentum. The energy from the sun has nothing to do with that. The process does generate its own heat, heat that doesn't come from the sun. Yes, this heat is radiated away, but not the momentum.

Heat can generate rotational kinetic energy, angular momentum in a closed system (no mass in/out, just heat energy in/out).

Kinetic energy yes. Momentum, no. An internal combustion engine gets its rotational kinetic energy basically by turning chemical energy into heat, and that heat into kinetic energy. But that doesn't change the momentum of the engine one bit. The angular momentum gained by a revving engine comes from external torque applied to it by the engine mounts.

[Jaaanosik]

...

Heat can generate rotational kinetic energy, angular momentum in a closed system (no mass in/out, just heat energy in/out).

Kinetic energy yes. Momentum, no. An internal combustion engine gets its rotational kinetic energy basically by turning chemical energy into heat, and that heat into kinetic energy. But that doesn't change the momentum of the engine one bit. The angular momentum gained by a revving engine comes from external torque applied to it by the engine mounts.

It is the other way around.
There is no external torque. If engine is not running then where is the external torque? Doing what exactly?
Nothing. No external torque. There is nothing external spinning the engine, is there?
The torque comes from the engine itself.
Why car engines have the torque as their specification?
It is action and reaction. The first action is engine running then the reaction is engine mounts 'counter-torque', preventing the engine block from spinning.

Here is the problem that might be in the way of agreeing what we say.
We start an electric motor on ISS, floating, stator and rotor spin opposite way.
The total net angular momentum is going to be zero.
Does the motor have an angular momentum as a system?
Would the system behave differently upon application of an external force when motor is not spinning compared to when the motor is spinning?

[Bored chemist]

Just for clarity for those that may not understand your point below is quite a good demonstration.

It's not a very good demonstration of how the Earth behaves, is it?

Heat can generate rotational kinetic energy, angular momentum in a closed system (no mass in/out, just heat energy in/out).

Would you like to demonstrate that?

Don't forget that I will be adding together the angular momentum of all the parts of the system before and after.

It is action and reaction.

Congratulation on spotting this.
It is the reaction against the Earth which provides the torque.
The effect is, perhaps, more apparent in helicopters which either need contrarotating blades or a tail rotor in order to fly.

Here is the problem that might be in the way of agreeing what we say.
We start an electric motor on ISS, floating, stator and rotor spin opposite way.
The total net angular momentum is going to be zero.

I already said that- weeks or months ago.

Does the motor have an angular momentum as a system?

No.

Would the system behave differently upon application of an external force when motor is not spinning compared to when the motor is spinning?

Not a well defined question.
Do you mean "if we took the wheel out of a child's toy gyroscope and replaced it with the motor with the its rotor going one way, and its stator going the other, would it act like a gyroscope?".
If so then the answer is no.
It wouldn't (neglecting friction, air resistance etc).

Because, overall, the combination would have zero angular momentum

Again, it's like having contrarotating blades on a helicopter (though that's not the only reason they do it).

[Halc]

I split this topic, since the vast majority of the discussion going on has no business being in the mainstream physics section. I pretty much put all the posts from those advocating violations of conservation of angular momentum in this topic.

Heat can generate rotational kinetic energy, angular momentum in a closed system (no mass in/out, just heat energy in/out).

Kinetic energy yes. Momentum, no. An internal combustion engine gets its rotational kinetic energy basically by turning chemical energy into heat, and that heat into kinetic energy. But that doesn't change the momentum of the engine one bit. The angular momentum gained by a revving engine comes from external torque applied to it by the engine mounts.

It is the other way around.
There is no external torque. If engine is not running then where is the external torque? Doing what exactly?
Nothing. No external torque. There is nothing external spinning the engine, is there?

If the engine isn't running, there is no change in angular momentum of the engine, so there cannot be external torque beinging applied. Do you think I said otherwise?

The torque comes from the engine itself.

An engine (revving or not) in a closed system cannot put external torque on itself, and thus cannot change its angular momentum one bit.  So if the momentum of the engine is changing, there must be an external torque being applied to it, and the mounts are the only place applying such a force.

The energy comes from the engine. The external torque comes from, well, external.
There are a few examples of engines running without mounts, such as the one on a Sopwith Camel. The drive shaft of the engine is bolted to the fuselage, with no actual connects to the engine itself.  In that case, positive torque is applied only through the drive shaft, making the momentum of the engine negative.  The propeller blades are bolted to the engine block which allows the air to apply positive torque to the engine preventing it from gaining even more negative angular momentum. The positive torque applied to the fuselage is countered by the negative torque via wing flaps.

Why car engines have the torque as their specification?

An isolated engine (a closed system) cannot deliver any torque at all. Nada. Car engines have a torque spec because they're expected to be mounted. A torque spec is pretty useless in isolation. My leaf blower, if hooked to the right transmission, can deliver more torque than a Jaguar engine. A power spec seems a more appropriate measure of an engine's ability to do work.

It is action and reaction. The first action is engine running then the reaction is engine mounts 'counter-torque', preventing the engine block from spinning.

Very good. So why is it "the other way around" then? Counter-torque is torque, not a different thing with different physical rules. That torque applied by engine mounts changes the angular momentum of a revving engine, something that wouldn't change if it was in a closed system.

Here is the problem that might be in the way of agreeing what we say.
We start an electric motor on ISS, floating, stator and rotor spin opposite way.
The total net angular momentum is going to be zero.
Does the motor have an angular momentum as a system?

You just said it has none, so no. This is consistent with your scenario, since no external torque has been applied, only energy.

Would the system behave differently upon application of an external force when motor is not spinning compared to when the motor is spinning?

It perhaps vibrates when running. It is hard to hold on to when running since the motor is spinning, but any application of identical external force on the stopped motor must have the identical effect on the running one. Same change to linear momentum, and same change to angular momentum. For instance, the running motor will not exhibit any gyroscopic effects since it has zero angular momentum.

[Bored chemist]

I pretty much put all the posts from those advocating violations of conservation of angular momentum in this topic.

Should this stuff be in teh "that can't be true" forum?

[Halc]

Should this stuff be in teh "that can't be true" forum?

There's lots of crap that should go there first then. Dave's threads are high on the list, but at least he had the sense to post here from the beginning, and not post his assertions in the main sections.

The thread title doesn't explicitly call out momentum, but I put the posts here from people that at some point assert non-conservation stances. I probably should have entitled it "Can energy affect..." rather than 'heat'.

Energy, or windmills, or any tall tower for that matter, can affect the spin rate of a rotating thing, so it isn't entirely 'not true'. A windmill spins, and thus absorbs more than its share of the angular momentum of the system.  The tower on which it stands changes the moment of the system. None of these touch the system angular momentum, but they do wiggle length of day by a few picoseconds or so.


On a totally unrelated note:  Three of us are all coming up on posting milestones:
Alan is close to 10K, you to 20K, and me to (just) 2K.

[Bored chemist]

Perhaps  Alan will come back and either explain where the torque is from, or admit that he was wrong.
Perhaps he won't.

[Jaaanosik]

We have been here before.
We discussed it, never came to a conclusion a few months back.

First thing to recognize is the difference.
The 3 CD players at 0:49s when the CDs are not spinning inside.
There is an external impulse to the system of connected 3 CD players when they are not running.
The system starts to rotate/spin.
The system center of mass moves/translates at velocity v1.

The CDs are spinning inside at 0:55s.
There is an external impulse (it appears to be the same magnitude, but off center)  to the system of connected 3 CD players when the are running (CDs are spinning inside).
The system does not start to rotate. The system wobbles.
The system center of mass does not move/translates at velocity v1 (magnitude).
The system center of mass is resisting the straight line motion/translation.

Do we have an agreement that the system demonstrates different attributes, the system behaves differently due to the internal motion of inner parts even thought the total net angular momentum is the same before the external impulse?

[Bored chemist]

(Am I The only one who worries about anyone in zero G who thinks that taping 3 CD players together  at right angles is ter than taping them together with the axes aligned?)

Anyay, you are missing the point, and I suspect it's deliberate.

Imagine that the CD player has an IR remote control and is running on batteries.

You set it in front of you in zero G then, using the remote control, you press "play".

What happens?