i seem to having trouble with questions related to elastic collisions. Please help me out if you can please. 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. What is the speed of the 0.21 kg ball just after the collision?