A 4 kg steel ball strikes a wall with a speed
of 11.4 m/s at an angle of 30.5 degrees
with the normal to the wall. It bounces oﬀ with the
same speed and angle. If the ball is in contact with the wall for 0.315 s, what is the magnitude of the average force exerted on the ball by the wall?
Answer in units of N

Question

A 4 kg steel ball strikes a wall with a speed
of 11.4 m/s at an angle of 30.5 degrees
with the normal to the wall. It bounces oﬀ with the
same speed and angle. If the ball is in contact with the wall for 0.315 s, what is the magnitude of the average force exerted on the ball by the wall?
Answer in units of N

jamesj · Answer

Change in momentum = (Force)(time)

because
Force = ma and a*time = change in velocity

and 
Change in momentum = m*(change in velocity)

====

Hence for this problem, calculate change in momentum.  You also have time.  Hence you can solve for force.