1. The problem statement, all variables and given/known data An object at rest in deep space explodes into two parts converting some of its original mass into kinetic energy of the fragments. Both fragments move at a speed of 0.60 c relative to the position of the original object. Each fragment has a rest mass of 3.0 kg. Calculate the rest mass of the original object. 2. Relevant equations Conservation of momentum m0v0 = m1v1 + m2v2 Mass-energy relationship E=mc^2 3. The attempt at a solution Kinetic energy is not conserved since explosion is inelastic. This is when I get stuck: how can I represent the kinetic energy ('lost mass') in the form of momentum?