Question

A ball with a mass of 0.30 kg hits the wall at a speed of 5.0 m/s and bounces back at a speed of 4.0 m/s.

Calculate the change in ball momentum.

Answer 1

$$\Delta p=m(v_f-v_i)=0.30\ kg\cdot(-4.0\ \dfrac{m}{s}-5.0\ \dfrac{m}{s})=-2.7\ \dfrac{kgm}{s}.$$

The sign minus means that the change in ball momentum directed away from the wall.

Answer:

$\Delta p=2.7\ \dfrac{kgm}{s},$ away from the wall.