Internal Forces and Work.
I. Internal forces always occur in action-reaction pairs.
II. The sum of work done by internal forces on a rigid body is always zero.
III. The sum of work done by internal forces on a non-rigid system (like a spring-mass system) can be non-zero.
IV. Internal forces cannot change the total momentum of a system.
V. Internal forces cannot change the total kinetic energy of a system.
