Hello.
Ive run into a problem with my understanding of systems and energy.
Ive thought the if a system is to change its energy, then work must be done
to the system (from outside the system) to change the energy of the system.
I hit a ball. I do work ti the ball giving it energy.
I have an example i cannot resolve.
The system is a person holding weights on the standard spinning chair. When
the person pulls the weights in, the moment of inertia decreases and the
rotational kinetic energy increases. All forces are internal. No external
work is done to change the kinetic energy of the system.