[Jaaanosik]

...
However the Earth is , for all practical purposes, rigid.
...

We started to talk about the Sun's energy input.
That's the point when we should stop thinking about the Earth as a rigid body.
The boundaries...
When we consider the Earth's crust as the most visible part of a bigger system then the OP question can be understood as 'do windmills affect the Earth's crust rotation?'

The answer to the OP is clear, yes.
If the rotation of the crust changes then something happens to the stuff beneath (mantle, ... ) to compensate and to keep the total angular momentum conserved (ignoring the stuff outside the crust: water, ...)

[Jaaanosik]

...
 I can also apply an external force or torque (and change the respective momentum) without application of external energy, so the momentum is simply not a function of energy input.

Well, it depends, ...
... who is looking, the reference frame and the boundaries.
'The momentum is a function of energy input.' is a true statement as well.

Also, applying an external force or torque by contact is applying an external kinetic energy.
Applying an external force or torque by field requires a question where is the field coming from.
Still, the momentum could be a function of energy input as well.
I am not sure what you were trying to say.

[Bored chemist]

The bold part, I am talking about three axes of rotation.

I tried to explain why that's not helpful with the comments about the cat.

You can resolve the rotation of the earth onto any set of axes you like.
But, as long as you maintain the same set of axes, the projections of the angular momentum  onto those axes will remain constant because the angular momentum remains constant/

There's a lamp behind me.
If I turn round the lamp is in front of me.
That's not because the lamp has moved, it's because I have changed the reference.

Like the cats rotation, applying energy within the system can be a cause of such rotation.

As I pointed out, technically, you don't need to supply energy to do this.

However, what you fail to take account of is that the cat's angular momentum is constant throughout the process.
The cat never spins. Bits of it do, and other bits spin the opposite way.
If the Earth did that, we would be dead,

[Bored chemist]

We started to talk about the Sun's energy input.

Yes we did.
God knows why.
It's not got anything to do with rotation.

[Bored chemist]

The answer to the OP is clear, yes no.

FTFY

[Bored chemist]

'The momentum is a function of energy input.' is a true statement as well.

OK, I take a rock and heat it up, thereby changing its energy.
Does it somehow suddenly have to start spinning or moving through space?
No, obviously not.
So neither angular nor linear momentum is a function of energy.
Just as well really, since they are vectors, not scalars.

Also, applying an external force or torque by contact is applying an external kinetic energy.

Really?
And there was me thinking I could make an electric motor.


Will you please think about stuff before you post it.
Just try to remember that everything you post will be looked at critically by people who do science for a living.
So, you should really try to imagine what our reaction to a post will be and, if that reaction is to trivially show that you are clearly wrong, don't post it.

You should be checking your own work before you post it, otherwise you may mislead other people who look at this to think that there's actually a controversy here.

[Bored chemist]

Quote from: Bored chemist on Yesterday at 21:48:15
Quote from: Bored chemist on Yesterday at 17:25:59
I will stop posting this when you either show that it was easy as you claim, or accept that it's impossible because the laws of physics prevent it.

Quote from: Bored chemist on 05/10/2020 18:31:40
Now, once again, since you said it was easy...
Quote from: Bored chemist on 24/09/2020 19:01:18
Quote from: Jaaanosik on 24/09/2020 18:51:25
It is easy to show that adding energy to a rigid body can change the axis of rotation,
Then show it.
But no cheating.
You have to do it without applying a torque.
Only add energy.



You keep on saying you can perform this miracle and that it's easy.
Why don't you do it?

Still waiting...

[Jaaanosik]

The bold part, I am talking about three axes of rotation.

I tried to explain why that's not helpful with the comments about the cat.

You can resolve the rotation of the earth onto any set of axes you like.
But, as long as you maintain the same set of axes, the projections of the angular momentum  onto those axes will remain constant because the angular momentum remains constant/

There's a lamp behind me.
If I turn round the lamp is in front of me.
That's not because the lamp has moved, it's because I have changed the reference.

The relativity says the lamp moved for the non-inertial observer.
There is a dr/dt change for the non-inertial reference frame.

Like the cats rotation, applying energy within the system can be a cause of such rotation.

As I pointed out, technically, you don't need to supply energy to do this.

However, what you fail to take account of is that the cat's angular momentum is constant throughout the process.
The cat never spins. Bits of it do, and other bits spin the opposite way.
If the Earth did that, we would be dead,

The bold part, I am saying the same thing, components change, the total sum of angular momentum is conserved.
I said the same thing with the ice can example.
Yes, people die, when a bad earthquake happens, hence the importance of the boundary conditions, space and time.

[Jaaanosik]

We started to talk about the Sun's energy input.

Yes we did.
God knows why.
It's not got anything to do with rotation.

It can generate a flip, as per the ice can example.
So the flip is not a rotation now?

[Jaaanosik]

'The momentum is a function of energy input.' is a true statement as well.

OK, I take a rock and heat it up, thereby changing its energy.
Does it somehow suddenly have to start spinning or moving through space?
No, obviously not.
So neither angular nor linear momentum is a function of energy.
Just as well really, since they are vectors, not scalars.

Also, applying an external force or torque by contact is applying an external kinetic energy.

Really?
And there was me thinking I could make an electric motor.


Will you please think about stuff before you post it.
Just try to remember that everything you post will be looked at critically by people who do science for a living.
So, you should really try to imagine what our reaction to a post will be and, if that reaction is to trivially show that you are clearly wrong, don't post it.

You should be checking your own work before you post it, otherwise you may mislead other people who look at this to think that there's actually a controversy here.

You keep pulling my quotes out of context.
It is impossible to have a reasonable discussion.

For example: "Also, applying an external force or torque by contact is applying an external kinetic energy."
How you came to conclusion that I am saying an electric motor cannot be made?
Where is the electric motor coming from? Why?

[Bored chemist]

So the flip is not a rotation now?

It's not a change in angular momentum nor does it give rise to a rate of rotation.

It can generate a flip, as per the ice can example.

Not with the earth.
Also, it's important to recognise that the energy added does not cause the flip.
The change from a solid to a liquid does that.

There are materials which liquefy when cooled.

You could spin a warm can of that in zero gravity and as it cooled it would give out energy and start to change the axis of rotation as it lost energy, cooled and "melted".

The change isn't caused by energy, it's caused by a change in the moment of inertia.

It's true that moving rocks around to turn the earth into a huge donut shape would change the rotation rate and need energy.
But it's not the energy that changes the rotation rate.

[Bored chemist]

Where is the electric motor coming from? Why?

You can get one on amazon. That's hardly the point.
An electric motor spins the rotor by contact- that's why they have brushes.
And yet it does not add kinetic energy to the rotor.
The motor just sits there; it has no kinetic energy to transfer.

Also, applying an external force or torque by contact is applying an external kinetic energy.

is incorrect- as shown by an electric motor which applies a torque, but does not apply external kinetic energy.

[gem]

Hi all
So Halc in regards to your 1kg mass moving at 2m/s you need to do that change in temperature and change in momentum Calculations after a collision. Else you give the impression momentum/ LoD value Is unaltered after frictional coupling.
Also Halc where you stated:

“Sound waves carry energy but not momentum.“

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

BC you stated:

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

Not sure that’s correct either

https://en.m.wikipedia.org/wiki/Crankshaft

So in regards to symmetry yes the sound waves leaving the angular Momentum motion sum to zero but the Total value of them individually is subtracted from the angular momentum total.

[Bored chemist]

Not sure that’s correct either

That's OK, I am sure it is.
What I'm puzzled about is why you think that something so obvious could invalidate a well known law of physics.

Would you like to speculate on that?
Here's a clue.
Imagine building a petrol engine with all the cylinders + spark plugs and valves etc.
But not including the bearings etc that hold the crankshaft in place.
Would it work?

[Bored chemist]

So in regards to symmetry yes the sound waves leaving the angular Momentum motion sum to zero but the Total value of them individually is subtracted from the angular momentum total.

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.

[Jaaanosik]

...
You could spin a warm can of that in zero gravity and as it cooled it would give out energy and start to change the axis of rotation as it lost energy, cooled and "melted".

The change isn't caused by energy, it's caused by a change in the moment of inertia.
...

Wooow
The energy melted the ice in the ice can example.
The primary factor is energy. There would not be any change of the moment of inertia if not for the energy.
I said that pages ago, it appears you are starting the grasp it now.
Starting... the last step is to agree the energy is the primary factor.
... and this is a domino effect.
The change of the moment of inertia leads to the change of the angular velocity around that one specific axis.
The change of the angular velocity is the angular acceleration.
The right hand rule and there you go ... there is a rotation around another axis.
The components are changing but the total sum of the angular momentum is conserved.

[Jaaanosik]

Where is the electric motor coming from? Why?

You can get one on amazon. That's hardly the point.
An electric motor spins the rotor by contact- that's why they have brushes.
And yet it does not add kinetic energy to the rotor.
The motor just sits there; it has no kinetic energy to transfer.

Also, applying an external force or torque by contact is applying an external kinetic energy.

is incorrect- as shown by an electric motor which applies a torque, but does not apply external kinetic energy.

Look, please, start reading the posts and stop shooting from your hips right away.
I said this:

Also, applying an external force or torque by contact is applying an external kinetic energy.
Applying an external force or torque by field requires a question where is the field coming from.

Also, applying an external force or torque by contact is applying an external kinetic energy. - like a hamster wheel.
Applying an external force or torque by field requires a question where is the field coming from. - like the electric motor.
Your example of the brushes and contacts has nothing to do with what I meant by contact.

This is a physics basics question.
What are the options to transfer kinetic energy?
The answer: contact or field.
Note: the contact is 'field' as well; when we zoom in down deep... friction, contact is a field as well, just atoms are very close to each other and they interact through their field.

[Jaaanosik]

Not sure that’s correct either

That's OK, I am sure it is.
What I'm puzzled about is why you think that something so obvious could invalidate a well known law of physics.

Would you like to speculate on that?
Here's a clue.
Imagine building a petrol engine with all the cylinders + spark plugs and valves etc.
But not including the bearings etc that hold the crankshaft in place.
Would it work?

Do you remember 'boundaries'? Think about them when you watch this video.
https://www.khanacademy.org/science/high-school-physics/torque-and-angular-momentum/conservation-of-angular-momentum/v/ball-hits-rod-angular-momentum-example

[Bored chemist]

Your example of the brushes and contacts has nothing to do with what I meant by contact.

I am sorry for thinking that you meant contact when you said contact.

The energy melted the ice in the ice can example.

And energy moving in the opposite direction would have the same effect in a can of thermosetting material.

So energy going in has the same outcome as energy going out.
That pretty much proves that it' not the transfer of energy that makes the difference.

I can even manage the same thing with a vanishingly small energy transfer.
Do you know how they make liqueur chocolates?


So. we can get the can to change from one stable state to another  by
(1) adding energy
(2) removing energy
(3) doing neither.
and yet you still think it's energy that changes it.

I don't think many would agree with you.

The change of the moment of inertia leads to the change of the angular velocity around that one specific axis.

And, as I said, if you rebuilt the planet and turned it into a donut you would alter the rate of rotation.
But windmills don't do that, do they?
So it's irrelevant.

Also, applying an external force or torque by contact is applying an external kinetic energy

That would be relevant if someone put a big spanner on the Earth or something and applied a torque.

Since nobody is doing that , it's irrelevant.
Why mention it?

This is a physics basics question.
What are the options to transfer kinetic energy?

Do you think the Sun is transferring kinetic energy ?

Or is that just more  gish gallop?

Do you remember 'boundaries'? Think about them when you watch this video.

I'm not the one who forgets the boundary.
Unless a force acts from outside the earth (i.e outside the boundary) the angular momentum within the boundary (i.e. of the Earth) will remain the same.

[Halc]

Sorry, I've not been able to keep up, and there's a lot of nonsense that doesn't warrant a reply, but I'll chime in for some blatant wrong assertions:

I can also apply an external force or torque (and change the respective momentum) without application of external energy, so the momentum is simply not a function of energy input.

Well, it depends, ...
... who is looking, the reference frame and the boundaries.
'The momentum is a function of energy input.' is a true statement as well.

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.

This is what I mean by momentum not being a function of energy. The first question would have an answer if it was. Sure, energy can be employed to exert an external force on said system, but a boat load of energy isn't going to make that mass change its momentum by itself. NASA would love a reactionless drive, but such a thing violates Newton's 3rd law. They have space ships with plenty of energy, but they can't use it to propel the ship. I can build a reactionless drive with the physics gem describes. He should patent it.

Also, applying an external force or torque by contact is applying an external kinetic energy.

Also, applying an external force or torque by contact is applying an external kinetic energy.

You need to read reply 320 where B-C asks you to consider the most simple counterexamples before posting (and even repeating) blanket statements like that, although admittedly I didn't didn't think the electric motor thingy illustrated the point clearly.

My example is a simple precessing wheel which has external torque applied to it by physical contact (visualize a top spinning at a tilt on a level table) yet has zero kinetic energy applied to it.  Its KE will remain unchanged except for losses to friction, which don't exist in an ideal environment, but the external torque applied necessitates its perpetual change in angular momentum.
For linear momentum, a ball twirling on a string is a nice example, which has force continuously acted on it by contact with the string, necessarily changing its momentum, yet without gain or loss of kinetic energy relative to frame F in which the center of rotation is stationary.

So Halc in regards to your 1kg mass moving at 2m/s you need to do that change in temperature and change in momentum Calculations after a collision. Else you give the impression momentum/ LoD value Is unaltered after frictional coupling.

No, the post-collision momentum calculation does not involve any thermal calculations. You simply add the momentum of the two objects together.  As simple as A+B.  Geez, I learned this when I was 13 in middle school intro physics class.

for example, you can not convert linear momentum into angular momentum

Not sure that’s correct either

https://en.m.wikipedia.org/wiki/Crankshaft

You do realize that a crankshaft spins due to an external torque being applied to it, not due to it converting its own linear momentum to angular momentum, right?  No, apparently you don't realize even this. Unbelievable.

You are here not to learn but simply to contradict at all costs rather than admit that you didn't know something.