Internal forces can change

Choose the correct statement regarding a system of particles. Internal forces can change the kinetic energy of the system without changing system's momentum O Impulse can be defined as a very large force that acts for a very short time and causes finite change in momentum. Internal forces can change the potential energy of the system without changing system's momentum Internal forces can change the mechanical energy of the system without changing system's momentum.

If only an external force can change the momentum of the centre of mass of an object, how can be internal force of an engine accelerate a car?

If only an external force can change the state of motion of CM of a body then how does the internal force of the brakes bring bicycle to rest?