Mass, m=200g=0.2 kg
Initial velocity, u=40 m s−1
Final velocity, v=0
Time, t=0.04 s
Initial momentum, p1=m×u=0.2×40=8 kg m s−1
Final momentum, p2=m×v=0.2×0=0
Change in momentum, p=p2−p1=0−8=−8 kg m s−1
Average force=m×(v−u)t=0.2×(0−40)0.04−200N
The negative sign shows that the force is applied in a direction opposite to the direction of motion of the ball.