# Dumbbells collision

Two identical dumb-bells are placed so that their ends are at (−d, +2l), (−d, 0), and (+d, 0), (+d,−2l). Each can be considered as two point masses m joined by a weightless rod of length 2l. Initially they are not rotating, and move with velocities (+V,0), (-V,0) so that the top of one hits the bottom of the other. If the coefficient of resitution is e then work out the centre of mass velocity and the angular velocity of each rod after the collision.

I tried doing this by energy and angular momentum conservation, which I think I can do if it's an elastic collision, but I'm not sure how the coefficient of restitution affects that. Any help would be greatly appreciated.

## Answers and Replies

tiny-tim
Science Advisor
Homework Helper
I tried doing this by energy and angular momentum conservation, which I think I can do if it's an elastic collision, but I'm not sure how the coefficient of restitution affects that.

Hi jsmith12! (When in doubt, try wikipedia … http://en.wikipedia.org/wiki/Coefficient_of_restitution)

In collisions, momentum and angular momentum are always conserved … so it's only the energy equation you have to change when the collision isn't elastic! 