Two identical steel balls, each of mass 4.3 kg, are suspended from strings of length 30 cm so that they touch when in their equilibrium position. We pull one of the balls back until its string makes an angle θ = 65° with the vertical and let it go. It collides elastically with the other ball. How high will the other ball rise? img:http://i242.photobucket.com/albums/ff106/jtdla/prob01a-1.gif I have a few ideas as to how to tackle this: To get the velocity one ball travels couldn't I do this: mgh+0=1/2mv^2+0 (for the ball on the left) Then could I put that velocity into a momentum equation. My second idea was wouldn't the ball that is just sitting there go up to the same height as the other one's initial height. I remember something being demonstrated with so little motor cars when we were talking about collisions.