1. The problem statement, all variables and given/known data There are two particles with rest masses m1 and m2. In the S frame they have velocities u1 and u2. Consider another frame, which we usually denote as the S0 frame, which has a velocity v in the S frame in the direction of increasing x (i.e., it is going to the right in the S frame). Show that the invariant (or rest or proper) mass of the system of two particles has the same value in the S and S0 frames (i.e., it is invariant under a Lorentz transformation). Recall that the system in the S frame has E = E1 + E2 and p = p1 + p2. 2. Relevant equations Really not too sure, I was thinking the Lorentz transformation for mass, but I am uncertain how to begin this question. Any tips to how I can begin, that would be greatly appreciated. Thanks.