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PF Gold
P: 693
 Quote by RoyLB [...] Is energy just a mathematical fiction / convenience, like imaginary numbers (OK - perhaps I opened a can of worms with that statement :-) or is there some physical "reality" to it? [...]
In the case of nuclear physics the nuclear binding energy makes a difference. The mass of a nucleus is not just the sum of the constituent protons and neutrons, the internal energy has inertial mass too.

In the case of a very large nucleus, such as a Uranium nucleus, there is so much repulsive force between the protons that the nucleus is teetering on the brink of falling apart. A neutron absorbtion event can trigger nuclear fission. The combined mass of the two fission products is smaller than the mass of the original Uranium nucleus.

In a sense the Uranium nucleus is under very large internal stress. There is the strong nuclear force that acts to keep the nucleus together, and the Coulomb repulsion between the protons acts to make the nucleus fission. This internal stress is a form of potential energy, and it has a corresponding inertial mass. The magnitude of that inertial mass is given by the following expression:

$$m = \frac{E}{c^2}$$