1. The problem statement, all variables and given/known data Two balls with masses m1 and m2 moving on a horizontal table in the same direction but at different speeds collide elastically. Obtain expressions for the position and motion of the centre of mass in terms of the separate positions and motions of the two balls before they collide. Take it that at the instant of observation they are passing x = x1, x2 respectively with speeds v1 and v2. How would your expressions simplify if m2 >> m1, while v1 and v2 are not very different. If m2 >> m1 and v2 << v1, what happens in the collision? 2. Relevant equations Xcm= M1X1 + M2X2/M1+ M2 3. The attempt at a solution Well, for separate positions the cerntre of mass should be same as above equation. But what about just before the collision?