If we study the motion of large number of spheres, it will be observed that two hard spheres do not attract each other, but rebound immediately on collision. That is they, do not come closer than their diameter ‘d’. The interaction potential ‘V’ for a pair of hard sphere is
d = diameter
r = distance of interaction of 2 spheres
It shows that there is infinite repulsion for r = d and no potential for r>d and hence repulsive forces binds them together.
