Question

A ball X of mass 1 kg travelling at 2 m/s has a head-on collision with an identical ball Y at rest. X stops and Y moves off. Calculate the velocity of Y after the collision.

Answer 1

According to the law of conservation of momentum, the two momenta given are equal to each other.
Before collision, momentum of the first ball is = (1)(2) = 2 kg.m/s
Before collision, momentum of the second ball = (1)(0) = 0

After collision, momentum of the first ball = (1)(0) = 0
After collision, momentum of the second ball is = (1)(v)
Now,
2 + 0 = 0 + 1v
⇒ v = 2
The velocity of Y after the collision is 2m/s.