Consider a gold nucleus to be a sphere of radius 6.9 fermi in which protons and neutrons are distributed. Find the force of repulsion between two protons situated at largest separation. Why do these protons not fly apart under this repulsion?
Given:
Largest Distance of Separation = Diameter of the nucleus ,
Charge on each proton , 
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Formula used:
By Coulomb’s law, the electric force is given by:
Where ϵ0 is the permittivity of free space
k is the electrostatic constant
q1 and q2 are the magnitude of charges
r is the distance of separation between the charges
The electric force is repulsive and is given by:
The protons do not fly apart because there is also strong nuclear force which is attractive and balances the repulsive electric force.