1. The problem statement, all variables and given/known data Two bodies of mass M and m separated by a distance 'R' are released and they move towards each other under the influence of gravity . What is the work done by internal forces (considering the 2 bodies as a system) when they are at a distance 'x' apart ? 3. The attempt at a solution When they are at a distance R apart their potential energy = -GMm/R . When they are at a distance x apart their P.E = -GMm/x. So change in P.E = GMm(1/x-1/R) . That should be the change in K.E = Work done by the internal conservative force. But isn't work done by internal force always zero . Maybe , I am confused .