Answer
2.7 balls per second.
Work Step by Step
See the textbook regarding kinetic molecular theory. The tennis balls hitting the trash can lid are just like particles colliding with the inside walls of a box. As stated in the text, the average force equals the momentum change during one collision, divided by the time between collisions.
In this case, the average force balances the weight of the trash can lid. The collisions are elastic, so the change in momentum of a tennis ball is twice its incoming momentum.
$$F_{av}=M_{lid}g=\frac{\Delta p}{\Delta t}=\frac{2m_{ball}v_{ball}}{\Delta t}$$
Solve for the time between collisions.
$$\Delta t=\frac{2m_{ball}v_{ball}}{M_{lid}g}$$
The reciprocal of “seconds per ball” is the number of balls per second that strike the lid.
$$\frac{M_{lid}g}{2m_{ball}v_{ball}}=\frac{(0.50kg)(9.80m/s^2)}{2(0.060kg)(15m/s)}=2.7\;balls/s$$
