1. The problem statement, all variables and given/known data A particle with mass m and speed (4/5)c collides with a particle (with mass m as well, but speed = 0) un-elastically, and continues as one particle. I have to find the speed and mass for this "joined" particle. 3. The attempt at a solution Ok, first - conservation of momentum! 1) I use that gamma_1*m*(4/5)c + 0 = gamma_2*M*v 2) I also use gamma_1*m + m = gamma_2*M Is this correct? In the notes I took in class today, I wrote this for #1: gamma_1*m*(4/5)c + 0 = gamma_2*M - but I guess I wrote this one down wrong? Thank for you all in advance.