1. The problem statement, all variables and given/known data http://grephysics.net/ans/8677/20 So to do this, I solved for the total rest energy of both particles. The rest energy of Kaon + K = Rest energy of Proton Rest energy of Proton - Rest Energy of Kaon = K K = P^2/2m ((Rest Energy of Proton - Rest Energy of Kaon)*(2*Mass of Kaon))^1/2 = P P = mv/(sqrt(1-v^2/c^2) If I do this and do all the algebra correct and solve for v, will this method give me the correct answer? I got the wrong answer but I suck at numbers X_x. I realize after looking at the answers on the website this is a poor way to do this problem with GRE time constraints, I just want to know if this thought process is flawed or not.