1. The problem statement, all variables and given/known data Consider a neutron which decays at rest. Calculate the momentum of the electron in MeV/c when the proton is produced at rest. (You may assume that the anti-neutrino has zero mass). 2. Relevant equations Relativistic momentum = [itex]\gamma(mv)[/itex] 3. The attempt at a solution Since the neutron is at rest, it has zero momentum. So I'm guessing that the electron has an equal and opposite momentum to the anti-neutrino...but if it has zero mass, how do I work out the momentum? I've also tried playing around with the equation [itex]E^2 = p^2c^2 + m^2c^4[/itex] but I still get equations involving the mass, which just give me back zero momentum. I'm also assuming that the electron doesn't just have zero momentum, otherwise it seems like a silly question..