1. The problem statement, all variables and given/known data Unstable, subatomic particles called muons have a rest energy of 105.7 MeV and a speed of 0.994c. If a muon were to decay and produce an electron and a photon, what would be the momentum of the electron as measured by an observer in the muon's frame? HINT: assume that the electro goes in the +x direction, the photon goes in the -x direction, and the muon is at rest in its own frame. 2. Relevant equations E = mc^2 + K; p = gamma*m*v; numerous other forms of relativistic energy equations 3. The attempt at a solution By the hint, (muon is at rest) we can assume that the initial momentum is zero. By conservation of momentum, initial momentum = final momentum thus, momentum of the electron = momentum of the photon. This is as far as I can get. The posted answer is 52.8 MeV/c.