Well go on then, ghh, you've got my attention. What's your answer - and what's your agenda?

My answers are:

I agree with Ian - if it's a perfectly elastic collision, the answer is v.

And if it's perfectly inelastic, the answer is v/2 (although object b would arguably no longer be a separate entity - a perfectly inelastic collision would produce a new object which is an amalgamation of objects a & b, and the new object would have mass 2m and move at v/2)

These answers make the following assumptions:

- none of a's translational kinetic energy is converted into another form (e.g. heat/sound) - which is also a requirement for a perfectly inelastic/elastic collision

- there's no rotational energy, as Ian has already stated

- you're measuring everything from the point of view of an observer who remains stationary at the original point where b was at rest (i.e. not from an observer who moves with b, for example)

- the speed is low enough (relative to the speed of light) that relativistic effects can be ignored

- the objects are free from the influence of all other forces (e.g. object b isn't stuck firmly to the ground or something)

Interestingly, if object b WAS stuck immovably to the ground, then the answer would be 0 - from the point of view of an observer also standing on the ground. But for an observer in space, the answer would be everso slightly greater than 0. (I'll leave it for you to work out why that's the case![}:)])