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Potential Energy is Mass, Too!

Where did the mass "go" in the reaction we just discussed? The answer is that the BINDING ENERGY of the ²³⁵U nucleus is substantially less negative than that of the final products.

Remember that the gravitational potential energy between two massive bodies is zero when they are infinitely far apart and becomes more and more negative as they get closer together? [Lower gravitational potential energy for an object at a lower height?] Well, the STRONG NUCLEAR FORCE that binds nuclei together has at least this much in common with gravity: it is attractive (at least at intermediate range) and therefore produces a POTENTIAL ENERGY "WELL" into which the constituents "fall" when we make up a nucleus.^24.8

The other thing to realize is that potential energy counts in the evaluation of the total relativistic energy of an object; and if the object is at rest, then its potential energy counts in the evaluation of its REST MASS. As a result, we might expect the rest mass of a space ship to be slightly larger after it leaves the Earth than it was on Earth, simply because it has left the "gravity well" of the Earth. This is the case! However, the mass change is imperceptibly small in this case.