The question: Suppose two identical particles, each with mass m and kinetic energy T, collide head-on. What is the kinetic energy of one in the rest system of the other? The solution: Is given on pg 109 of Griffiths' Introduction to Elementary Particles. Griffiths writes down the total 4-momenta of the system in the lab frame and in the CM frame. The zeroth component in the CM frame is 2E/c. So I guess that E is the energy of one of the particles in that frame. The zeroth component in the lab frame is E'+mc^2, however. Why is mc^2 added for the lab frame? Why not have 2E'/c as in the CM frame? Thank you!