Q. Two masses traveling at the same speed v0 collide head on. Nothing is known about the collision other than that it is not completely inelastic.

After the collision the larger mass 2m moves at speed v in its original direction of motion, while the smaller mass m recoils at half its original speed.

What is the speed v?

1. v = v0/3
2. v = -v0
3. There is no way to say since we don't know how elastic the collision was.
4. v = v0/4
5. v = v0/5
6. The larger mass continues to move at the same speed: v = v0.