question

[Seif]

If you have 2 objects that collide...A & B....and you have the mass and initial velocity of both, and the final velocity of A. What would be the better way of solving for the final velocity of B? Using KEi=Kef or Pi=Pf?

[HallsofIvy]

Since you are asked for velocity- a vector quantity, you will need to use "conservation of momentum". Energy is a "scalar" (a number) and while conservation of energy could tell you the speed of B, it can't tell you the direction.

If everything is in a straight line, it's still better to use conservation of momentum to get "left" or "right" correct.