A 0.25 kg ball is attached to a 26-cm piece of string. The ball is first raised so that the string is taut and horizontal, then the ball is released so that, at the bottom of its swing, it undergoes an elastic headon collision with a 0.21 -kg ball that is free to roll along a horizontal table. Find the velocity of each ball after collision. since its elastic i know Ke is conserved as well as momenta. So i have 2 equations. Im not sure what to do with those 2 equations to get the final velocities of each ball. My teacher said something about frame of reference.