# Can internal forces change the kinetic energy of the system?

According to me, they should not because negative of work done by external agent = change in potential energy and since, due the absence of external forces, total energy remains conserved therefore, kinetic energy of system should also remain the same.

• Here's a counterexample: the system is a mass on a spring on a rough block. Friction is an internal force between the block and the mass. If the mass is set in motion, it will eventually stop. Apr 24, 2018 at 5:53
• What about a system of two moving masses colliding inelastically? Apr 24, 2018 at 5:53
• physics.stackexchange.com/questions/206436/… Apr 24, 2018 at 5:56
• I think I have understood.Did you mean that in case of man in boat energy stored in man (a kind of potential energy of man boat system)has got converted into kinetic energy of the man.Similarly in my case maybe due to movement of particles in the system some bond may break or form changing the potential energy of the system resulting in change in kinetic enrgy. But still one question persists in my mind that isn't it declaring formula , negative of work done by external agent = change in potential energy wrong? Apr 24, 2018 at 6:19

The work done by the internal forces can increase the kinetic energy of the body. In accordance to work-energy theorem, the net work done by the forces acting on a body is equal to the change in the kinetic energy of the body. The work done by internal forces will increase the internal energy of a body. The increase of internal energy of the body results, its constituent atoms or molecules might acquire this energy in several different forms like increase of translational kinetic energy and increase of rotational kinetic energy. Therefore, the work done by the internal forces can increase the kinetic energy of the body

• physics.stackexchange.com/questions/214848/… Apr 24, 2018 at 5:59
• please refer this, the internal forces are not included in work energy theorem. Apr 24, 2018 at 6:00
• It might be worth noting explicitly that you are implicitly assuming usual factorization into energy of motion of the CoM and energy of motion relative the CoM, and that you arguing (correctly) that internal work can effect that latter but not the former. Apr 24, 2018 at 17:05
• @PalakJain "the internal forces are not included in work energy theorem" You will find that there is some argument on that matter. It is useful to exclude internal work before you have built up the framework for factorizing the energy of a system, but the theorem can (and should) be applied to all work on a system. Apr 24, 2018 at 17:08

For sure internal forces cause a change in the potential energy of the system. But as for kinetic energy, it may or may not change it. For example, consider a ball raised to a height h above the ground slowly, (considering our system is the earth and the ball), the ball gains potential energy and all the mechanical energy gained is potential. Ok then when it is falling down it will have both changes in kinetic and potential energy.