1. The problem statement, all variables and given/known data Consider an elastic collision of two particles in the centre of mass frame. Briefly explain why the speed of EACH particle after the collision is the same as before the collision. (FYI this is exam revision so it isn't worth any marks) 3. The attempt at a solution The centre of mass itself has the same direction, velocity, and momentum after the collision, but I do not understand why the speed of each individual particle (in the COM frame) is the same after the collision. The individual speeds certainly change from the lab frame, and given that the COM frame does not change at all after the collision, the speeds of individual particle speeds should change too, right?