1. The problem statement, all variables and given/known data A particle of mass M decays at rest into a massless particle and another particle of mass m. The magnitude of the momentum of each of these relativistic particles is: 2. Relevant equations E = γmc2; p=γmv 3. The attempt at a solution Pf-Pi = 0; but the particle is at rest, so pi = 0; and the momentum of massless particle = 0; unable to start up with exact condition. Tried equating initial and final energies also, but not getting the right answer. Please help.