Hello I've been trying to get confirmation, or otherwise, that the total mass of an atom can be considered to be equal to the sum of the masses of the individual atomic particles it contains plus the mass equivelent of the internal energy, (PE+KE), of the particles. I have also been trying to find a reference to a work that refers to this question. In my searches I came here to PF and read the FAQ ........"What is the mass-energy equivelence". The FAQ pointed out that E and m are equivelent when p = 0, this being when internal energy is part of mass. So far so good but it gives the example of a box containing hot springs being more difficult to push than a box containing cold springs and that's where I get stuck. I sort of understand why the hot box might seem to be more massive but I don't see how that can be justified by assuming that p=0. A main difference between the hot and cold springs is that in the latter case each particle has greater energy and this alternates between PE and KE. For each particle separately p = 0 only during those instances that it comes momentarily to rest. A main problem I see is that the equation relating E, m, p and c relates to a single particle and to kinetic energy. How does it relate to an assembly of particle whose movements are somewhat random with respect to eachother and how does it relate to potential energy? Thank you.