1. The problem statement, all variables and given/known data Two particles of mass 4 and 5 kg (respectively) move towards each other. If the mass of the system is 20kg what can you say, qualitatively and quantitatively about the system in the center of mass frame? 2. Relevant equations In the CM frame net momentum is zero: gamma1*m1*v1 + gamma2*m2*v2 = 0 mass of the system: gamma1*mass1 + gamma2*mass2 = 20 3. The attempt at a solution I know that this problem with these two equations and two unknowns is solvable, but the algebra is extremely long and difficult. Is there a more elegant, short way to solve the problem? Perhaps using E^2 = (pc)^2 + (mc^2)^2 ?