# About elastic and inelastic collision

1. Nov 8, 2012

### KFC

I am studying the collision from the text, in which they said inelastic collision is the case when two collide objects stick together after collision. But in the class, the instructor said two objects stuck together is not the sufficient condition for inelastic collision, we should figure out if the kinetic energy conserved or not. So which one is right and why?

2. Nov 8, 2012

### Staff: Mentor

Neither, if you've quoted your instructor correctly (but I'd bet a fair amount that he didn't say "sufficient", he said "necessary", in which case he's right).

An elastic collision conserves kinetic energy; an inelastic collision does not.

If the colliding objects stick together, that's a sufficient but not a necessary condition for the collision to be inelastic.

3. Nov 8, 2012

### Staff: Mentor

In other words, if the objects don't stick together, the collision may be either elastic or inelastic, depending on whether KE is conserved or not.

We sometimes call the case where the objects stick together "completely inelastic", and the cases where they don't stick together but KE is not conserved "partially elastic" or "partially inelastic".

4. Nov 8, 2012

### KFC

Thanks guys for your explanation.