Momentum Problems

Question

1) How great of an impulse is needed to change the speed of an 8 kg cart from 32 m/s to 50 m/s? How many seconds would you need to apply a 10 N force in order to cause this impulse?

2) On a serve, a tennis ball may leave the tennis racket of a top player at a speed of 65 m/s. If the ball has a mass of 0.06 kg and it is in contact with the racket for 0.03 seconds, what is the average force applied to the ball? Would that be enough force to lift an average size person (75kg)? Assume that the tennis ball is initially not moving.

Answer 1

I will set up the solution for the first question, and then you try to answer the second question by applying the same kind of reasoning.

(1)

impulse = change in momentum

I = mv₂ - mv₁ = m(v₂ - v₁) = (8 kg)[(50 m/s) - (32 m/s)] = ? kg m/s

impulse = force x time

I = Ft ⇒

t = I/F

Plug in 10 N for F and the result for I from above and get t in seconds.