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We know that energy is created during nuclear fission and there is a loss in mass. But every body possess potential energy even when it is at rest (where height = radius of earth). So during nuclear fission, due to loss in mass, should there be loss in potential energy of the body as P = mgh ?

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  • $\begingroup$ Total energy is a conserved quantity, including the rest mass energies. The fragments will have less gravitational potential energy, the differences will appear as kinetic energies of the fragments. $\endgroup$ – anna v Feb 7 '18 at 7:30
  • $\begingroup$ @annav I still don't understand how the less gravitational potential energy will appear as the kinetic energy of the fragments. Can you please explain it? $\endgroup$ – Vedansh Agrawal Feb 7 '18 at 8:46
  • $\begingroup$ Energy is conserved, but in nuclear physics this energy sum includes the difference in rest masses from before and after. Doing the sum after, there are more potential energies which add up to less than neutron+uranium potential energy, but each fragment carries enough kinetic energy so that the sum of all energies, kinetic , potential and rest masses adds up to the same number as before the fission. $\endgroup$ – anna v Feb 7 '18 at 8:49
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Once you start considering relativity (which you have to do for mass-energy interconversions) the split of energy into kinetic and potential energy becomes ill defined. So you are quite correct that if you take the usual definition of gravitational potential energy:

$$ V = -\frac{GMm}{r} $$

then any event in which the total rest mass changes will cause a change in the potential energy. But that's OK because potential energy isn't a conserved quantity. The conserved quantity is the total energy and that does remain constant.

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