1. The problem statement, all variables and given/known data Ok so im given the masses of D meson, pi meson and K meson and told that a D meson at rest decays into a K meson and pi meson. Told to calculate the energy and velocity of the pi meson.. 2. Relevant equations 3. The attempt at a solution well i know you have to use conservation of 4 momentum.. so energy before = total energy after..and i know that the momenta of the pi and K meson after the collision are equal and opposite, but i cant see how to use this to get total energy of pi meson..:S any tips?