>>7611071Imagine a pair of objects connected in two ways: a spring pushing them apart, and a pair of very strong magnets pulling them together.
Initially, these objects are very far apart, so there's an enormous amount of potential energy stored in the separation of the magnets.
However, the spring is very stiff, and so at long distances where the magnetic attraction is very weak it takes a lot of effort to push the objects together.
But, the magnetic attraction gets stronger and stronger as you push them together, faster than the spring force increases. Eventually, you reach a threshold distance where the magnets overwhelm the spring and snap together, slamming the two objects together and releasing energy in the process.
If the magnets are powerful enough, and the spring weak enough, the energy released will overcome the energy spent compressing the spring to this threshold in the first place and a net gain of energy will take place.
The spring, in this analogy, is the mutual electrical repulsion of the positively charged nuclei.
The magnets are the Strong Nuclear Force, which is extremely strong but (unlike gravity and electromagnetism) has a finite range: it decays so rapidly with distance that it is effectively nonexistent unless you are extremely close to the nucleus. In fact, very large atoms become unstable because the strong force is failing to reach even the other side of the nucleus!
(And on the other hand, if the magnets are weak enough compared to the spring, then if the two objects are already stuck together applying just a little bit of energy to pry the magnets apart could allow the spring to overcome the magnets and expand, driving the objects further apart and releasing all its stored energy. This is how fission works.)
