I have a question about the true meaning of E=mc^2. For starters, a certain amount of mass (m) can be converted into an amount of energy (mc^2), or vice versa. For example, two antiparticles can annihilate and leave only energy behind. So it is mass-energy that is a conserved quantity, not each one alone. That being said, it is my impression that mass is still one thing, and energy is another, albeit that they can be converted into each other. Mass has ties to inertia and gravity that energy does not. For example, consider a system, something like a uranium nucleus, that spontaneously decays. It is said that "the mass of the nucleus before the fission is greater than the sum of the constituent parts". This basically accounts for the potential energy (that manifests itself as kinetic energy upon fission) as extra rest mass. But to quote the rest mass of the uranium nucleus as including this potential energy does not make sense to me. Will the nucleus in fact behave as though it has this extra mass, for example if I apply a force to it (hypothetically) and measure the acceleration?