The graph of potential energy of a pair of nucleons as a function of their separation shows a minimum potential energy at a value r (approx. = 0.8 femtometers). Below r the PE is positive (actually rises sharply from a negative to a positive value). Above r the PE is negative, and becomes zero beyond a certain value of r. For a separation greater than r the force on the nucleons is attractive, while for separations less than r the force is strongly repulsive. I would appreciate if someone could explain the above phenomenon to me. Thanks.