KE Loss Qs 8: Elastic Collisions Explained

[yolo123]

8. All KE is lost when:
(a) two bodies of equal mass collide,
(b) two bodies of opposite initial velocity collide,
(c) two bodies of equal mass and opposite initial velocity collide,
(d) never.

None of these make sense to me.
(a) Billard balls have elastic collisions.
(b) Billard balls again disprove this...
(c) Billard balls...
(d) ...... Elastic collisions exist.

 

[zeralda21]

Kinetic energy is lost in inelastic collisions. Does that help? Total energy is however conserved.

 

[yolo123]

Yes. I don't see how that helps though. My counter-examples still hold true.

 

[guitarphysics]

Your 'counterexample' on part (d) is incorrect; so what if elastic collisions exist? That tells us nothing about a case where *all* KE is lost, which is what the question is about.

As for the question itself, I'm not actually sure! . Are you completely sure it was "All KE is lost when" and not "All KE *can* be lost when"? That would make more sense, because in all the examples it can be conserved as well, depending on whether the collision is elastic or not. Or maybe I'm just delirious because it's midnight. Oh well.