# find momentum absorbed by wall after struck by ball

A tennis ball with a mass of 0.50 kg is thrown at a wall at a speed of 20 m/s. The ball hits the wall perpendicularly and bounces off in an elastic collision.

a) How much momentum does the wall absorb when struck by the tennis ball?

b) A tennis serving machine shoots tennis balls at the wall, as described above, at a rate of eight tennis balls per second. What is the average force on the wall?

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## SOLUTION This solution is **FREE** courtesy of BrainMass!

Because it is an elastic collision, the initial speed must be equal to the final speed, that is, the final speed is 20 m/s. In term of velocity, the final velocity is -20 m/s.

To calculate the momentum the wall absorbs when struck by the tennis balls, we can calculate the momentum lost by the ball.

âˆ†p=p_f-p_f

=mv_f-mv_i

=0.50 kgÃ—(-20m/s)-0.50 kgÃ—20m/s

=-20 kg m/s

Since the ball lost 20 kg m/s of momentum, the wall absorbs 20 kg m/s of momentum.

âˆ†p=Fâˆ†t=mâˆ†v

Fâˆ†t=20 kg m/s

F=(20 kg m/s)/âˆ†t

The tennis serving machine shoots 8 balls/second, so one ball is shot every 1/8 second = 0.125 second.

F=(20 kg m/s)/âˆ†t

=(20 kg m/s)/(0.125 s)

=160 N

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