Question

A ball with a mass of `6 kg` moving at `9 ms^-1` hits a still ball with a mass of `4 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

The total momentum before the collision, `54 kg ms^-1`, will be the same as the total momentum after it. When divided by the mass of the second ball this yields a velocity of `13.5 ms^-1`.

Explanation:

Momentum is the product of the mass of an object and its velocity, and it is conserved : the total momentum before the collision is the same as the total momentum after it.

`p=mv`

Before the collision, the `4 kg` ball has a velocity of `0 ms^-1` and therefore also has momentum of `0 kgms^-1`. That means all the momentum is carried by the `6 kg` ball.

`p=mv=6*9=54 kgms^-1`

After the collision the `6 kg` is stationary so its momentum is zero, and all the momentum is carried by the `4 kg` ball.

`p = mv`
`v = p/m = 54/4 = 13.5 ms^-1`