farolero said:
well then you could show to solution of the problen we could be discussing the meaning of words forever
You are the one who is insisting on discussing a particular word meaning to the point that you refuse to understand the analysis.
i can not understand how that a point in the tube that actually makes a circle have any revelance for solving the problem
I already explained that once. I will go over it again more slowly. In addition, you should realize that it is
your job to solve the problem, not ours. The Physics Forum way is to guide people to the solutions they seek rather than to simply spit out numbers.
Let us review.
We are trying to find a formula for the angular momentum of the two astronauts plus tube in terms of the mass of each astronaut (m), the angular rotation rate of the tube (##\omega##), the moment of inertia of the tube (I) and the current distance of each astronaut from the center of the tube (R).
I have asked you for such a formula. You have been unable to provide it.
We have reached agreement (I believe) about the angular momentum of the tube alone:
##L_{tube}=I \omega##
We have failed to come to an understanding about the angular momentum of each astronaut. I have a correct formula in mind and have offered a justification for it. You have rejected that justification, apparently out of a disagreement on the meaning of the word "tangential". So it seems that we must avoid the use of that word.
Your current question is how the fact that a point in the tube makes a circle could possibly be relevant to determining the angular momentum of each astronaut.
The astronaut has a velocity. I think you would agree with this.
As viewed from any inertial frame, that velocity is not in a straight line away from the center of the tube. Possibly you will agree with this.
Instead, that velocity is at some unknown angle. Except at the start of the situation, it has a non-zero component in the direction away from the center. We can call this its "radial component". It also has a non-zero component at right angles to that. We can call this its "clockwise component".
At any time while an astronaut is still in the tube, the "clockwise component" of his velocity will match the "clockwise component" of the velocity of the spot on the tube wall that he is passing at that time. This is an assertion with which you may agree after some thought.
We have an easy formula that will give the "clockwise component" of the velocity of a spot on the tube wall.
The "clockwise component" of the astronaut's velocity is important because it is, by definition, perpendicular to the displacement of the astronaut from the axis of rotation.
That means that it is also equal in magnitude to ##v \sin \theta## where theta is the angle that the astronaut's velocity makes with his displacement from the center.
The angular momentum of the astronaut is the vector cross product ##\vec{R} \times \vec{p}## where R is the astronaut's displacement vector and p is his momentum.
Factoring out the m from p=mv, that is also equal to ##m \vec{R} \times \vec{v}##
It is a basic property of vector cross products that this is equal in magnitude to ##mRv \sin \theta## where ##\theta## is the angle that the velocity makes with the displacement vector.
But ##v \sin \theta## is equal in magnitude to the component of the astronaut's velocity at right angles to his displacement.
And that is what we are calling the "clockwise component" of his velocity.
Which is, as above, equal to the "clockwise component" of the velocity of a spot on the wall of the tube at the astronaut's current position.
Which is given by ##\omega R##.
Which is why the circular motion of a point on the tube is relevant.
Edit: ##\sin \theta##, darnit.