Nuclear Forces

Physics recognises four fundamental forces of nature, and the one responsible for holding the nucleus together -- overcoming the mutual electrostatic repulsion between its positively charged protons -- is the strong force, also called the nuclear force. It acts between nucleons (proton-proton, proton-neutron and neutron-neutron alike) and is predominantly attractive at the separations found inside a real nucleus.

The nuclear force is still not completely understood theoretically, but its main experimentally established properties are well established. It is by far the STRONGEST of the fundamental forces at nucleon-scale distances -- roughly 50 to 60 times stronger than the electrostatic force between the same particles at the same separation. Unlike gravity and electromagnetism, whose influence in principle extends to infinite distance (just growing weaker with distance), the nuclear force has an extremely short RANGE: strong and dominant over distances of only a few fm, but falling essentially to zero once two nucleons are separated by much more than that. And it is CHARGE-INDEPENDENT: the nuclear force between two neutrons at some given separation is (experimentally) the same in strength as the force between two protons at that same separation, or between a proton and a neutron -- it does not distinguish nucleons by their electric charge, only by their being nucleons at all. …