I was reading that the reason the nucleus of the atom is so heavy is that there is so much energy being used to keep the nucleons together with the strong nuclear force, and energy=mass. I can understand that. But it is energy which equals mass, not force, right? The strong nuclear force is just exerting a force to keep the nucleons together, not energy. The force may translate potential into kinetic energy if it was actually doing work across a distance. But these nucleons aren't going anywhere. They are stationary. So why is there talk of energy?