Some of notable matters in derivation of `MvecA=vec(F)_(ext)`.

Explanation

(1) To determine the motion of the centre of mass no knowledge of internal forces of the system of particles is required for this purpose to know only the external forces.
(2) To obtain this equation we did not need to specify the nature of the system of particles. The system may be a collection of particles in which there may be all kinds of internal motion, or it may be a rigid body which has either pure translational motion or a combination of translational and rotational motion.
(3) Whatever is the system and the motion of its individual particles, the centre of mass moves according to `MvecA=vec(F)_(ext)`.