1. The problem statement, all variables and given/known data An unstable particle of mass M = m1 + m2 decays into two particles of masses m1 and m2 releasing an amount of energy Q. Determine the kinetic energies of the two particles in the ZMF. Given that m1/m2 = 4, Q = 1 MeV, and that the unstable particle is moving in the lab frame with kinetic energy 2.25MeV, find the maximum and minimum kinetic energies of the particle of mass m1 in the lab frame. 2. Relevant equations See below 3. The attempt at a solution Let the particle of mass m1 be A and of mass m2 be B. For the first part, pA* + pB* = 0 in the zero-momentum frame and so mAEA* = mBEB* where E is the kinetic energy of the particle in the ZMF. I think the other equation to use is EA* + EB* = Q but I'm not sure how to justify this. 1) Why should the energy released in the decay (in the lab frame) be equal to the sum of energies of the resultant particles in the ZMF? For the second part, not sure how to start or even why there are a range of possible answers. Seems to me that the kinetic energy of the unstable particle in the lab frame gives directly its velocity in the lab frame, which is thus the velocity of the ZMF; meanwhile, the ZMF kinetic energies of the two particles after the decay can be converted into ZMF velocities. Then we just do vlab = v* + vZMF to find the lab velocities of the two particles. These lab velocities can be converted to lab KEs, done. 2) How is there a range of kinetic energies of A possible, for the second part? How should I do it?