I've seen the solution to finding the velocity after a ball bounces off of a wall. http://www.lhup.edu/~dsimanek/ideas/bounce.htm If a ball of mass m1 and velocity v1 hit a massive wall, what is the final velocity. I understand the math. What I find a little confusing is reconciling the math and how the equation m1v1 = m1v2 + MV satisfies conservation of momentum if v1 = -v2 and V = 0. Is it that the massive wall of mass M moves will an infinitesimally small velocity such that MV balances the -2m1v2?