Question

A ball with a mass of `4 kg` moving at `6 m/s` hits a still ball with a mass of `8 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

`3m/s`

Explanation:

Conservation of momentum states that there must be the same amount of momentum before and after a reaction. Momentum is given by the equation

`p=mv`

where `p` is momentum, `v` is velocity and `m` is mass.

Before the reaction, momentum is

`p=mv`
`p=4kg*6m/s=24(kgm)/s`

After the reaction, there must the same momentum of `24(kgm)/s`. Since the first ball has stopped moving, it has transferred all of its momentum to the second `8kg` ball.

`p=mv`
`24(kgm)/s=8kg*xm/s`

Rearranging, we find that

`x=24/8m/s=3m/s`