Physicists have categorized all forces occurring in nature under gravitational, electromagnetic, strong nuclear and weak nuclear forces. Both gravitational and electromagnetic forces can be expressed as inverse square laws. But nuclear forces cannot be expressed as a simple formula. They are basically attractive and they keep the nucleons bound in a nucleus. The nuclear forces occur between nucleons. Nuclear forces are short ranged, effective within a distance of femtometre. This distance is called as nuclear range.
Nuclear force is not central force. Many of the nuclear properties may be understood on the basis of the properties of nuclear forces. Nuclear forces are spin dependant.
The origin of nuclear forces explained by Yukawa in 1935, under the name exchange theory of nuclear forces. According to Yukawa, nuclear forces arise due to exchange of particles between nucleons called p-mesons. p-mesons appear in three forms p+, p-and po. There is continuous exchange of a meson between one nucleus and the other. The neutrons and protons keep on changing into each other due to exchange of p-mesons between them.
The following equations show some of such exchange.
