1. The problem statement, all variables and given/known data A lambda particle decays into a proton (at rest) and a pion. The rest masses are: lambda: 1116 MeV/c^2 pion: 140 MeV/c^2 proton: 938 MeV/c^2 we want to find the energy of the a) pion b) lambda (before decay) 2. Relevant equations I am assuming we need to use the energy-momentum 4-vector equation E2=(mc2)2 + (pc)2 3. The attempt at a solution So I think we know in the lab frame that the momentum of the pion and the lambda are the same, since the proton is at rest. So the energy of the pion must be its rest mass plus the mass deficit from the lambda; 140+(1116-(140+938))=140+38=178 , but there must be some energy that is from the initial momentum of the lambda. Could there be a missing parameter? I am unsure how you would figure out how much energy would be from the momentum without any other information.