"Two balls of the same mass approach each other with a relative velocity of 10 m/s, what is their relative velocity after the collision, which is presumed elastic?" I assumed that the balls have equal and opposite velocities of 10 m/s. Should the individual velocities be 5 m/s and -5 m/s (because 10 m/s is the relative velocity)? When I solved the problem, the balls simply exchanged their velocities. I said that their relative velocities after the collision are 10 m/s. However, this is wrong because there is only one relative velocity. I think I have the concept wrong. Please clarify.