Describe the nuclear force and its characteristics. 

The nucleus is the central part of the atom. In nucleus positively charged proton and chargeless neutrons are present.

The coulomb repulsive forces between proton - proton act for all distance small and large however they are held tightly together in the tiny region like nucleus.

This shows that some other attractive force between the nucleons in the nucleus must be prevailing which should be large enough to overcome the effect of coulomb repulsive force and must be enough to hold them together.

In a nucleus force acts between two protons between two neutron and between proton-neutron is called nuclear (or strong) force.

The constancy of binding energy per nucleon can be understood in terms of its short-ranged.

Following are the features of nuclear force obtained from experiments between $1930$ to $1950$ .

$(i)$ The nuclear force is much stronger than the Coulomb force acting between charges or the gravitational forces between masses. That's why it holds protons and neutrons in the nucleus.

- Gravitational force is much weaker than even Coulomb force.

$(ii)$ The nuclear force between two nucleons falls rapidly to zero as their distance is more than a few femtometres. This leads to saturation of forces in a medium or a large sized nucleus, which is the reason for the constancy of the binding energy per nucleon.

A plot of the potential energy between two nucleons as a function of distance is shown in the figure.