NOTE: I AM NOT POSTING A HW QUESTION. It is the last problem, 4.29. 4.29 A "superball" of mass m bounces back and forth between two surfaces with speed Vo. Gravity is neglected a nd the collisions are perfectly elastic. a. Find the average force F on each wall. b. If one surface is slowly moved toward the other with speed V «v, the bounce rate will increase due to the shorter distance between colli- sions, and because the ball's speed increases when it bounces from the moving surface. Find F in terms of the separation of the surfaces, x. (Hint: Find the average rate at which the ball's speed increases as the surface moves.) Here is the link to the answer : http://physics141.uchicago.edu/2002/hw4.pdf This is a problem from Kleppner and Kolenkow. I have a problem with the working. It is claimed that after every collision, the velocity changes by 2V. My point is that initial velocity before striking the wall was Vo towards the left and after the collision, the velocity is Vo + 2V towards the right, thus making the change of velocity 2(Vo + V) and not just 2V. Can someone justify how it is that one can solve the problem as done above?