Why can fission and fusion both produce energy?

Описание

When a heavy atom becomes unstable, it undergoes fission and splits into two smaller atoms and release energy. In a reverse process, two smaller particles smack together and undergoes fusion, to become a heavier particle. This process also releases energy. How come to opposite reactions both release energy?

The thing that binds together nuclei is the strong nuclear force. But this interaction is short ranged, so it can kinda only talk with it neighbors. However, the protons are positively charged and so they show coulomb repulsion. With a high number of protons, this repulsion gets too large and so the nucleus can bind stronger by splitting (fissioning). A nuclei with a small number of protons, can bind stronger by pairing/fusion with other nuclei to bind stronger by the strong nuclear force.
This balance evens out at iron, where t can’t gain more energy by fusioning. That’s why it’s the stars die once they fused into iron.


This video is from a physics quiz try it here:
https://www.youtube.com/watch?v=WCMMwcmY0IA