# Friction and elastic collisions

1. Jul 24, 2012

### dorker

Someone told me I'd be more likely to get help here. Say you have these two balls moving in opposite directions. The balls float in the air and thus by themselves have negligible friction, but each is carrying a (detachable) bar across the ground, which has friction. On the very instant the balls collide with each other, they let go of their respective bar and get attached to the other ball's bar (see pic).

Would this collision be frictionless, and thus elastic, or not? I really could see it going either way.

2. Jul 25, 2012

### ehild

I assume that the balls collide, they are in contact during collision.
If the balls are in contact for negligible time the friction does not change the kinetic energy.
The process of releasing one bar and being attached to the other needs further consideration. I assume the ball and bar are fixed to each other, moving with the same velocity.Do the balls release their bars just before collision and attach to the other bar after they collide, or the exchange of the bars happens just after the collision? The exchange of bars can be considered as other collision, and as the ball and bar have to move together at the end, this part is inelastic in principle and changes the energy unless the attaching bar and ball have the same speed. .

ehild

Last edited: Jul 25, 2012