Example $8\mathrm{.}2$ You throw a ball with a mass of $0\mathrm{.}40$ kg against a brick wall. It is moving horizontally to the left at $30$ m$/$s when it hits the wall; it rebounds horizontally to the right at $20$ m$/$s$.($a$)$ Find the impulse of the net external force on the ball during its collision with the wall. $($b$)$ If the ball is in contact with the wall for $0\mathrm{.}010$ s$,$ find the average horizontal force that the wall exerts on the ball during the impact.