1. The problem statement, all variables and given/known data A partical of mass m ha speed 0.546c relative to inertial frame S. The particle collides with an identical particle at rest in the inertial frame S. Relative to S and in terms of c, what is the speed of S' in which the total momentum of these particles is 0. 2. Relevant equations 3. The attempt at a solution I can see that in the S' frame, Pa = - Pb so that means that Va = - Vb since both masses are equal. But I just can't get my head around why i need to use special relativity because everything is in reference to the S frame anyway.