A tennis ball moving at 18 m/s strikes the 45 degree hatchback of a car moving a

Question

A tennis ball moving at 18 m/s strikes the 45 degree hatchback of a car moving away at 12 m/s. Both speed are given with respect to the ground. The ball rebound elastically from the car. Assume that the car is much more massive than the ball, which is reasonable. a) Determine the speed of the ball relative to the car before the bounce. b) If the ball bounces elastically, how fast and in what direction will it be moving with respect to the car after the bounce? c) What is the velocity of the ball with respect to the ground after the bounce? Where did the lost kinetic energy of the ball go? d) Explain why in this problem it was safe to assume that the speed of the car is not noticeably affected by the ball striking it.

Explanation / Answer

a)The relative speed of ball wrt car will be:

Vr = (18 x cos45 - 12) = 0.726 m/s

Hence, Vr = 0.726 m/s

b)The ball bounces off with

v = 18 cos45 = -12.73 m/s opp to the direction of its intial motion.

c)The velocity of the ball wrt to ground is:

v = 12.73 m/s

collision is stated as elastic in the question hence no loss of KE.

d)Because the car is very very massive as compared to the ball.

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