Here's the problem: "Two steel balls are suspended on (massless) wires so that their centers align. One ball, with mass 2.30 kg, is pulled up and to the side so that it is 0.0110 m above its original position. Then it is released and strikes the other ball in an elastic collision. If the second ball has a mass of 3.10 kg, to what height does it rise above its original position?" I started by assuming that the change in upward momentum would equal zero, but I came out with the wrong answer. Do I need to incorporate potential and kinetic energy into this problem? If so, I'm unsure exactly how to do so.