Nuclear force
Is a short range force extending upto 10 fm. It is fifty-sixty times stronger than electromagnetic force. Nuclear force is independent of charge. Nuclear force between two protons is same as nuclear force between two neutrons or nuclear force between a proton and a neutron. It is not a central force. It cannot be solely determined by distance. It depends upon the spins of the nucleons as well.
Heisenberg in 1932 proposed exchange force theory. Yukawa extended this theory and calculated mass of p - mesons. According to this theory, proton does not remain proton forever and similarly neutron does not remain as neutron forever. They go on changing for instance.