This post relates into the idea of post: https://www.physicsforums.com/showthread.php?t=58918 I was talking to my physics teach and he said some thing confusing. I have a video of the ball drop and the separation; however, I do not have the clip RIGHT before the collision (with the earth). The clip jumps from: the two balls a bit above the table --> to the objects separated. My teacher said something along these lines: “You cant prove the change in momentum because you do not have the velocity right before the collision. You need to find the height of where the basketball would have bounced and find the loss of energy. To do so, calculate the loss of Potential energy with each bounce of the basketball after the collision. Find an equation with that data that will show the loss of potential energy. When you do, plug in the original height of the basketball before you released it and see where the basketball would have bounced. This loss of potential energy plus the initial kinetic energy of the tennis ball (after collision) plus the initial kinetic energy of the basketball (after collision) should equal the original potential energy of the system before you let go of the balls. This will allow you find the initial velocity right before the collision to find the change in momentum.” I’m not sure what he is trying to tell me. I might have got some ideas wrong but that’s the best I can remember from class. How does finding that all the energies add up to the initial potential energy give me the velocity of the two balls before they hit?