Let's assume that a system has zero total momentum. The following relationship between mass and energy should apply: [itex]E=mc^2[/itex]. If a system is overall at rest, does that mean that any internal changes to that system, assuming they leave the system with non-negative mass, will not be able to produce more work than mc2? The work is used for reaction with another system, initially placed sufficiently far away to be insignificant when the thought-experiment begins. To clarify the motivation for my question. I thought of energy as having unknown bias. The number and character of the acting phenomenon (e.g. fields), unknown and not essential to the application of the conservation law from classical mechanics. The total work in a a given period of time and spacial vicinity may (predominantly) involve only part of the entire range of acting forces. But, if the above formula expects the energy to be non-biased aggregate of the system's ability to do work, then adding the potential, kinetic, and field energies, for example, should always produce amount lower than the one predicted by the mass equivalence or would be incomplete/incorrect. Is that the case?