Hi Please have a look on the attachment. Suppose that a 0.1 kg rubber ball having velocity of 60 m/s is moving between two walls A and B, and the distance between the walls is 1 m. It is having elastic collisions with the walls. Let's focus on what is happening at wall B. The ball is moving toward wall B. The momentum of ball is 0.1 x 60 = 6 kgm/s. It strikes wall B, has elastic collision, and reflects back and starts traveling toward wall A. Let's assume that it spends an infinitesimal time, dt=10-3s, during the collision. The force exerted by the wall on ball is F=(mv-mu)/dt=(-6-6)/10^-3=-12000 N. The force exerted by the ball on wall B is 12000 N. After having a collision with wall B, the ball starts traveling toward wall A and it would cover a distance of 2 m before it has another collision with wall B. It will take almost 0.034 seconds to strike wall B again and the frequency of collisions with wall B is 30 collisions per second. The average force might measure almost 12000 N or little less than that on some sensor placed at wall B while this process of the ball movement between two walls continues. Do you agree with what I'm saying above? Thank you for the help.