Nuclear Force

Since like charges repel, and protons inside a nucleus are packed only a few fermi ($10^{-15}$ m) apart, simple electrostatics predicts an enormous mutual repulsion between them. A quick estimate confirms this: at a separation of $10^{-15}$ m, the Coulomb repulsive force between two protons works out to about 230 N, giving each proton an acceleration of about $1.4\times10^{29}\ \text{m s}^{-2}$ - roughly $10^{28}$ times the acceleration due to gravity at the Earth's surface. If this electrostatic repulsion were the only force acting between nucleons, the nucleus would fly apart instantly.

Since real nuclei plainly do not fly apart, there must be a second, much stronger attractive force holding protons (and neutrons) together against this repulsion - this is called the strong nuclear force, whose properties were mapped out experimentally through the 1930s-1950s:

(i) Very short range, but the strongest force in nature. The strong nuclear force acts only over distances of a few fermi; beyond that range it essentially vanishes. But within that short range, it dominates completely - both the Coulomb repulsion between two protons and the gravitational attraction between any two nucleons are far weaker than the strong nuclear force acting between them at nuclear separations, making it the strongest of all the fundamental forces. …