I know that by definition a conservative force is a force that is independent of the path. An internal force is a force within a system. Now, the mechanical energy is conserved only when conservative forces are acting on a particular system. On the other hand some people say that it is conserved when the forces acting on the system are internal forces. From this I would think that internal energy = conservative energy. I was wondering what is the connection between the concepts of internal and conservative forces? What is the relation between the internal/external forces and conservative/non-conservative forces? .