Suppose we've got two still particles of any mass we would like, infinitely far apart. The total energy here is of course sum of their mass energies. Now if we bring them together so that one orbits around the other, what would be the total energy of this system? Perhaps, my guess, in such situation where there are only two still particles, without external force, since it is not possible for them to form an orbiting system (please check this for me, but I think it should be true), the change in total energy of the system will be only equal to the work done by the external force. Is that true?