# About friction and elastic collisions

1. Jul 24, 2012

### dorker

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?

EDIT: sorry. This is not for any homework, it's a question that occurred to me. Is it still considered a "homework type" question? If so, please move the thread.

Last edited: Jul 24, 2012
2. Jul 24, 2012

### tiny-tim

hi dorker!
friction has nothing to do with elasticity

if two cars are launched into the air, and then collide, then there's no friction, but the collision obviously isn't elastic

3. Jul 24, 2012

### dorker

^ You sure about that? An elastic collision is one where there's no kinetic energy lost. And friction is the main thing one would think it's lost to.

About your example, a car can't be properly modeled as a single body. A lot of energy is probably lost between the different pieces that compose it. I think the proper comparison would be two balls colliding in the air, which I imagine would be an elastic collision (provided they're not too fast, so air friction is negligible).

4. Jul 25, 2012

### tiny-tim

in a collision, the energy lost to friction is almost zero

most of the energy "lost" is in heat vibration noise and distortion
for energy and momentum equations?

of course it can​

5. Jul 25, 2012

### dorker

I see. Thanks.