Question

two billiard balls of equal mass undergo a perfectly elastic head-on collision. if one ball's initial speed was 1.90 m/s , and the other's was 3.20 m/s in the opposite direction, what will be their velocities after the collision?

Answer 1

One can use conservation of momentum and energy and obtain the result that the velocity of separation in such a collision will be equal to the velocity of approach

Thus, after collision their speeds will be 3.20 and 1.90 m/s

They will exchange velocities in the collision

If they were approaching each other their relative speeds are

5.10 m/s w.r.t. each other

If they were going in the same direction before the collision their relative speeds before and after the collision are 1.3 m/s

The problem here implies that their relative speed before the collision is 5.10 m/s (traveling in opposite directions)