[Bored chemist]

If the bearings are friction-less then CDs are not part of the system in this exact scenario.

That may be true in the case where the impulse is exactly tangential to the rotation axis, but it is not true in general.

Hence what I am trying to get across. The devil is in the detail.

And yet, when I Tried to nail down the details I got told I was

Escaping to unrealistic assumptions, really?

[Bored chemist]

If the bearings are friction-less then CDs are not part of the system in this exact scenario.

Seriously? Since when did friction define whether or not something was a part of a system or not?

Just think about this exact scenario and tell us how you can change the rotation of the CDs inside?

I can grab hold of the pair of players and rotate it so that the axis of rotation moves by 90 degrees.

[Bored chemist]

Rather Than us defining the system and you wriggling out of it, perhaps we could look at it the other way round.

Please  show us what impulse you could apply to the two pairs of players  which would give rise to them  following different motions

[Jaaanosik]

If we change time boundaries during the CDs acceleration then we need to specify ideal EM field (no skipping), again some unrealistic stuff.

It may be a language thing,but that makes no sense.

No EM field is involved.
Time isn't good at obeying boundaries.
What do you mean?

What happens when the external momentum would be applied at the time when the CD players are turned on and the CDs are accelerating.

[Bored chemist]

If we change time boundaries during the CDs acceleration then we need to specify ideal EM field (no skipping), again some unrealistic stuff.

It may be a language thing,but that makes no sense.

No EM field is involved.
Time isn't good at obeying boundaries.
What do you mean?

What happens when the external momentum would be applied at the time when the CD players are turned on and the CDs are accelerating.

Exactly the same as when they were stationary, or at full speed.
At all time the total angular momentum is zero.

[Jaaanosik]

Rather Than us defining the system and you wriggling out of it, perhaps we could look at it the other way round.

Please  show us what impulse you could apply to the two pairs of players  which would give rise to them  following different motions

One example, let us consider the there is a bearing friction.
The CD players are off.
There is a random transfer of the momentum to the CDs inside of the CD players.
Therefore the CD players will have random angular velocity each time the same external momentum is applied.
The same random transfer of the momentum to the CDs applies to the running CD players as well.

[Jaaanosik]

If we change time boundaries during the CDs acceleration then we need to specify ideal EM field (no skipping), again some unrealistic stuff.

It may be a language thing,but that makes no sense.

No EM field is involved.
Time isn't good at obeying boundaries.
What do you mean?

What happens when the external momentum would be applied at the time when the CD players are turned on and the CDs are accelerating.

Exactly the same as when they were stationary, or at full speed.
At all time the total angular momentum is zero.

We added an external momentum, we are analyzing a delta.
There is a change.

[Bored chemist]

By symmetry, whatever you added to one disk, you subtracted from the other, and so the effects cancel.

[Jaaanosik]

By symmetry, whatever you added to one disk, you subtracted from the other, and so the effects cancel.

No, that's my point about the random friction. There is no symmetry.

[Bored chemist]

By symmetry, whatever you added to one disk, you subtracted from the other, and so the effects cancel.

No, that's my point about the random friction. There is no symmetry.

Two things.
First, there's no friction in the system under discussion,
Second, random stuff cancels out on average.

[Halc]

If the bearings are friction-less then CDs are not part of the system in this exact scenario.

Nonsense. This violates momentum conservation laws.  I can build a reactionless spaceship drive if this ludicrous statement is true.

Two things.
First, there's no friction in the system under discussion,
Second, random stuff cancels out on average.

Secondly, there's no random stuff in a classic system.  Formulas for coefficient of friction do not involve probabilities.

Any friction in a CD player is countered by a motor keeping the CD rotating at a constant relative speed, required for the music to play properly.

[Jaaanosik]

By symmetry, whatever you added to one disk, you subtracted from the other, and so the effects cancel.

No, that's my point about the random friction. There is no symmetry.

Two things.
First, there's no friction in the system under discussion,
Second, random stuff cancels out on average.

You asked in #395, I answered in #398, friction is considered.
Do you agree with a delta per instance?

[Jaaanosik]

If the bearings are friction-less then CDs are not part of the system in this exact scenario.

Nonsense. This violates momentum conservation laws.  I can build a reactionless spaceship drive if this ludicrous statement is true.

Please, check the setup. How do we spin the CDs that are on the friction-less bearings?
The external momentum applied to the CD players as per the image.


Wait, wait, wait, ... I claimed that first.
Do you remember this?

[Halc]

How do we spin the CDs that are on the friction-less bearings?

By the CD player applying torque to the CD.

The drawing you posted looks a bit like a diagram of automobile disk brakes, which isn't very representative of a frictionless system.  No bearings are depicted at all that I can see.

You continue to post complications to a simple scenario, seemingly in attempt to either obfuscate the fact that your physics doesn't work, or to resist giving a direct answer to any question. You're a troll. You don't want to learn, you only want to contradict.

[Bored chemist]

Any friction in a CD player is countered by a motor keeping the CD rotating at a constant relative speed, required for the music to play properly.

To be fair, I wrote that out of the discussion by saying we were considering a model where friction must be so small you can ignore it.

On the other hand, the thread is about the earth and atmosphere where friction - like the wind - is pretty much random.

[Halc]

To be fair, I wrote that out of the discussion by saying we were considering a model where friction must be so small you can ignore it.

If there's a motor compensating for friction, even very high friction can be ignored, since it is internal and generates only heat, not net torque. But the motor in the CD player cannot be ignored.
The CD player will spin the CD at a constant relative RPM, which is different than a box with a frictionless wheel in it and no motor.  The latter will have a smaller moment along the spin axis.  So if I spin the CD player in the direction the CD is spinning, the motor has to add RPM to the CD to compensate.  The motor performs work.  That's different behavior than a box with a wheel in it.

That said, strap two CD players face to face an despite the two motors doing work, you cannot tell if the things are running or not.  The internal torque by the two motors always cancel each other and there is no external effect.

Not so with the 3 players strapped orthogonally, which behaves differently than three boxes with free spinning wheels inside.  Keep that in mind.  Jano of course has no clue what he's talking about and has not explicitly spelled out a single scenario with an explicit statement about the difference in behavior between a pair of zero-momentum devices running or not. He's adding needless complication in effort to never have to actually make such a statement.

[Jaaanosik]

Let's go slowly.



Two CD players taped together, CDs inside, not spinning, friction-less bearings.
The external momentum as per the image.
What happens?
The CD players rotate, the CDs inside do not rotate.
Any translation?

[Bored chemist]

The external momentum as per the image.

This phrase does not make sense in English,

[Bored chemist]

He's adding needless complication in effort to never have to actually make such a statement.

That's why I'm going for frictionless bearings and disk that coasts.
It's one more thing he can't talk bollocks about

Jano of course has no clue what he's talking about and has not explicitly spelled out a single scenario with an explicit statement about the difference in behavior between a pair of zero-momentum devices running or not.

Ditto.

That said, strap two CD players face to face an despite the two motors doing work, you cannot tell if the things are running or not.  The internal torque by the two motors always cancel each other and there is no external effect.

I know that.
You know that .
Reality knows that.
But the more idealised we make the system, the fewer things he can hide behind, so...
let's solve the problem for the spherical horse in a vacuum.
(and then watch him explain how the 6000 fold  bigger angular momentum that the disk has because of the subatomic particles,doesn't make a difference because... magic)

[Halc]

Let's go slowly.

Slowly is probably less needed than being precise, and you are being anything but in this post.

Two CD players taped together, CDs inside, not spinning, friction-less bearings.
The external momentum as per the image.

Two CD players taped together (or for that matter, any system of components, connected or not) can only have one angular momentum vector and one linear momentum vector. I can only guess that the picture shows one of each, but lacking labels, I'm left guessing.

So let's say the top vector is the angular momentum vector, which means the assembly is tumbling forward (rolling away from the point of view (PoV).  If that's the case, the CDs are also spinning with the players just like the seatbelted occupant of a tumbling car must rotate with the car. So saying the player has angular momentum but the disks do not (are not spinning) is a contradiction.  It could be done if the player angular momentum was parallel to the disk rotation axis, but the picture shows a vector perpendicular to it.

The lower vector is then the linear momentum vector, which means the thing is moving to the right.

What happens?
The CD players rotate, the CDs inside do not rotate.

Impossible.  The CDs are not free to remain stationary if the case is tumbling like that.  Perhaps you need to attempt a more precise description of what you have in mind.  It would also help if you say if the CD players are facing the same way or opposite each other. The picture shows only one of them, not two taped together.

Any translation?

The vector at the bottom indicates a translation to the right. If I guessed wrong which was which, then the top vector indicates a translation to the left, and the bottom vector indicates rolling towards the PoV.

I would think for simplicity you'd have the thing start with zero linear momentum, but in fact you didn't specify it at all except for the hints from the unlabeled vectors in the picture.  If they mean different things, you should make them different colors. If you meant them both to be angular momentum, then it's wrong since an object can have only one angular momentum vector.