# Homework Help: 2 Cylinder elastic rotational collision

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1. Jul 18, 2016

### redivider

1. The problem statement, all variables and given/known data

A cylinder with mass 3kg slides on ice with its base surface at 5m/s and collides with an identical but stationary cylinder. The collision is elastic. After the collision, the center-masses of the cylinders move at angles 45 and 30 degrees from the starting direction, What is the rotational energy of the first cylinder?

Ok so the problem I am having here is that I cannot picture the problem and that is why I am asking here as there is no sketch in the book. I do not understand how the angles are set up.

2. Relevant equations

Is it any of these? :
I'm feeling pretty dumb, bear with me .

3. The attempt at a solution

Not yet attempted.

2. Jul 18, 2016

### Orodruin

Staff Emeritus
Yes. One of those.

Now, two of them are impossible (think about which and why) so only one remains.

3. Jul 18, 2016

### redivider

Well the collision happens at an angle not head-on so I believe it is the 2. one, and the angle under which the first one hits the second one is, I believe (45+30)/2 and then I pretty much have the solution, just the "muscle exercise" remains.

4. Jul 19, 2016

### haruspex

I see no suggestion that anything is rotating.
Not sure what you mean by that.

5. Jul 19, 2016

### redivider

Imagine a pool ball getting hit, thats what I think this problem is similar to. The collision happens like so: http://i.imgur.com/htf6M8X.png

EDIT: I don't know how to explain it in english I guess, this is what I meant: https://vid.me/DVHd

Last edited: Jul 19, 2016
6. Jul 19, 2016

### haruspex

Sure, but
• I do not know what you are defining as the "angle under which the first hits the second"
• The acquisition of rotational energy depends on the friction of the surfaces, which is not specified.

7. Jul 19, 2016

### Orodruin

Staff Emeritus
Well, the angles are given so you can use conservation of momentum to find the velocities and conservation of energy and angular momentum to find the rotations. This is a question based on conserved quantities, not forces and torques.

8. Jul 19, 2016

### haruspex

Ok, I was forgetting that the definition of elastic collision means that not only is no energy lost to the elastic deformation of the materials, but none is lost to friction between contact surfaces either.