1. The problem statement, all variables and given/known data A K_0 particle has a mass of 497.7 MeV/c^2. It decays into a -∏ and +∏, each having a mass of 139.6 MeV/c^2. Following the decay of the K_0, one of the pions is at rest in the laboratory. Determine the kinetic energy of the other pion after the decay and of the K_0 prior to the decay 2. Relevant equations E = γmc^2 = mc^2 + K 3. The attempt at a solution This is the exact wording of the problem from my text book. I feel like in a chapter meant to be an introduction to special relativity they would specify whether we are talking about rest mass's or relativistic masses. The semi-general consensus amongst my peers is to do the problem as if all mass's given are rest masses. Kinetic energy of K_0 particle before decay: (γ-1)mc^2 = Kinetic Energy and the total energy is γmc^2 Since the total energy before the decay is γmc^2, and energy has to be conserved in all reference frames, in the reference frame of the lab where one of the ∏ particles is at rest I think I can put Rest mass energy + kinetic energy of K_0 particle = rest mass energy of both ∏ particles + the kinetic energy of one of them. So now I have two equations and three unknowns, being γ and the two kinetic energies... This may not be the most clearest thing I've ever typed but special relativity isn't exactly clear in my head either X_x.