Internal forces within a system can alter

Internal forces in a system occur in equal and opposite pairs according to Newton's third law of motion. Therefore, the net internal force on the system is always zero.

Since the net internal force is zero, it cannot change the total linear momentum of the system.

However, internal forces can do work on the particles within the system. For example, during an explosion or when a compressed spring between two masses is released, internal forces convert potential energy into kinetic energy, thereby increasing the total kinetic energy of the system.

Thus, internal forces can alter the kinetic energy but not the linear momentum of the system.