What happens to PE during elastic/inelastic collisions?

[southernson]

Elastic/Inelastic collisions are always defined in terms of KE, it being conserved in elastic and not in inelastic. But what happens to PE? In horizontal applications there's usually no PE involved, but consider a ball dropped onto a surface that experiences an inelastic collision. The ball won't reach the same height, so would one say that PE was not conserved? I've also had it explained to me that the PE immediately before the collision is the same as it is immediately after the collision, so PE is conserved.

 

[tiny-tim]

collisions are "instantaneous"

I've also had it explained to me that the PE immediately before the collision is the same as it is immediately after the collision, so PE is conserved.

Yes, the difference in PE before and after is either zero or negligble.

Examination questions about collisions assume that a collision is "instantaneous".

In other words, the collision takes such a negligibly short time that you can regard the time as zero.

In zero time, the height doesn't change, so the PE doesn't change (or, if you prefer, in a negligibly short time the PE changes a negligible amount, so you can ignore it).

 

[kkrizka]

The ball won't reach the same height, so would one say that PE was not conserved?

Yup, there are ways to loose energy in our non-perfect reality. For example, energy can be lost via sound or heat.