Question

A ball with a mass of `4` `kg` moving at `7` `ms^(-1)` hits a still ball with a mass of `7` `kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

Before the collision, the first ball has all the momentum (since the second is stationary). `p=mv=4xx7=28` `kgms^(-1)`.

Momentum is conserved. After the collision the second ball has all the momentum. `v=p/m=28/7=4` `ms^(-1)`.