As we know that acceleration of center of mass is due to only external forces acting on a system of particles and internal forces have no contribution in acceleration of center of mass.
Due to internal forces acting on a system of particles, individual particles of system can accelerate and
their momentum can change but overall as both action and reaction of an internal force is acting inside
the system of particle on different particles, internal forces do not effect on motion of center of mass.
To understand the concept of internal forces on a system of particles and motion of center of mass
follow the video – https://youtu.be/pwCsdFWzAYc
The basic application to understand is that center of mass motion remain unaffected by the internal
forces of system however due to internal forces and their work kinetic energy of all particles of a system
may increase but these particles move in such a way that center of mass motion remain same as before.
To grasp the application of this concept in various problems, see the example videos below –
