Consider the problem of returning a serve in tennis. A serve is hit at a speed of 63 m/s and is hit back to the server with a speed of 40 m/s. 

(a) What is the impulse imparted to the ball? 

(b) What is the approximate average force on the ball (m = 57 g)? For simplicity, assume one-dimensional motion; that is, assume the serve and the return travel along the same line.

You will need to estimate the collision time, that is, the time two objects are in contact during a collision. For comparison, the collision time for a baseball–bat collision is approximately 0.001 s, while the time for a tennis racket–ball collision is about 0.01 s. In general, hard things produce short contact times and soft things lead to long contact times.