What is nuclear force?

It is a force which is responsible for holding the nucleons ( proton and neutron ) in a nucleus and thus making the nucleus stable.

It has four types:
(1). n-n force
(2). p-p force
(3). p-n force
(4). n-p force

Main characteristics of Nuclear Force:-
(1). Nuclear forces are attractive forces and strongest among all the other forces.
(2). Nuclear forces are short range forces.
(3). Nuclear forces are operative within the range of  10¯¹³ cm. 
(4). Nuclear forces are central forces.
(5). Nuclear forces don't obey inverse square law

(6). Nuclear forces are charge independent.
(7). Nuclear forces are spin dependent. e.g.
      A complete of a proton and a neutron with parallel spin form a stable particle (deutron).
(8). Nuclear forces turn into strong repulsive forces when the distance between
       the nucleons becomes smaller as compared with nucleus diameter.

Assumptions:-
(1). The nuclear force ( n-n , p-p , n-p ) are assumed to be almost equal in magnitude and
       independent of charge of nucleons & supported by ( p-p , n-p ) scattering at lower energies.
      This assumption is known as "Hypothesis of charge independence of nuclear forces".
(2). The nuclear forces ( n-n , p-p ) are assumed to be nearly equal and must be smaller
       than ( n-p ) forces & supported by ( p-p , n-p ) scattering at high energies.
       This assumption is known as "Hypothesis of charge symmetry of nuclear forces".