A 'body' is a collection of particles. Therefore Newton's laws can be applied to individual particles in a rigid body then summed up over a body to see how they relate to the motion of a body.
A general body:
If Newton's second law is applied to particle i the following is obtained:
If the results of this for all particles are added together this becomes:
The sum of
the mass of all particles is given by 
,
which is in turn equal to the total mass of the system.
The position of the center of mass, G is given by 
, where
Therefore:
The total external force is given by the sum of all the individual external forces:
All the internal forces cancels in pairs
Therefore
we are left with:
It can be seen that
Newton's second law 'F=ma' applies to
any rigid body provided it is applied to the center of mass
any rigid body provided it is applied to the center of mass