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I am aware that during nuclear fusion for light elements and nuclear fission for heavy elements, the resultant elements have less mass than the original reacting elements (ie the mass defect) because of the differences in their binding energies per nucleon. This is what allows them to release energy.

Now, from my understanding from reading online, this mass defect comes into existence because of the strong nuclear force — some of the mass of the nucleons is used up to form the strong nuclear force to keep the nucleus intact.

So, if the mass defect is essentially equal to the binding energy of the nucleus, and this energy is equal to the strong nuclear force holding the nucleus together, why is this energy released as usable energy after the fission/fusion? Isn't this the energy keeping the nucleus intact? How can it be released and still leave the nucleus under the influence of the strong nuclear force?

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    $\begingroup$ If gravitational potential is essentially gravity holding the planet together, why is it released when we drop a book onto the floor? $\endgroup$ – Jon Custer Apr 11 at 21:45
  • $\begingroup$ @JonCuster when you rose the book, you had to use energy to do that work against gravity. That energy you gave then manifested into potential energy in the book, which then turns to kinetic energy when you dropped it. So you gave something (work) and got kinetic energy out of it in the end. In the case of binding energy, the nucleons are giving their mass and getting the strong nuclear force - the binding energy. So then, why is there usable energy in the end? Shouldn't it just be a conversion of mass to strong nuclear force with no left over energy to cause heat? $\endgroup$ – Main Man Andy Apr 11 at 21:52
  • $\begingroup$ The nuclei that can fission split into pieces further down the binding curve - nature made them in a way that they can fall to the ground, and that energy needs to be released. Similarly, fusing (some) nuclei allows them to form a nucleus more tightly bound, so again, you release the potential energy. $\endgroup$ – Jon Custer Apr 11 at 21:57
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Only 1% of the mass of the original nucleus is released as the fission energy, mostly in form of the kinetic energy of the fission elements that fly apart at 3% of the speed of light.

The remaining daughter nuclei have 99% of the mass of the original nucleus, but the daughter nuclei are still intact meaning they still have strong interaction inside them that keeps them in one piece.

Please see this answer:

Why is energy released during fission reactions?

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