I was thinking just a moment ago and fragments of random knowledge and ideas gathered to make this question... If I remember my college physics class correctly, an evidence of the mass-energy equivalence is that in an atom, the sum of the rest masses of the nucleons is greater than the rest mass of the nucleus. The missing energy is being used by the field, i.e. it serves as a medium for the nuclear (strong?) force to act between the nucleons. Is it true that the mass missing is precisely equal, via [itex]E=m_0c^2[/itex], to the nuclear potential energy of the system?? Side Q: Does "nucleon" has a plural? Ethimologically, where do the peculiar "plural endings" such as "1 nucleus, 2 nuclei", "1 radius, 2 radii",etc. come from? Greek?