1. The problem statement, all variables and given/known data A particle with mass m has speed 0.802c relative to inertial frame S. The particle collides with an identical particle at rest relative to frame S. Relative to S and in terms of c, what is the speed of a frame S' in which the total momentum of these particles is zero? This frame is called the centre of momentum frame. 2. Relevant equations 3. The attempt at a solution If particle 2 is at rest in the S frame, then in the S' frame particle 2 must have the same velocity as the S' frame relative to the S frame, but I'm not sure how to apply this.