Consider a case where a person stands on top of a rotating disk. The disc is given to rotate at a constant rate. There are two possible movements of the man:

1.He moves away from the center: In this case moment of inertia increases and hence the angular velocity decreases. The net mechanical energy of the system decreases.
2.He moves towards the center: In this case the moment of inertia decreases, therebey causing an increase in the angular velocity. The net mechanical energy increases.

In both the cases, the man expends some energy to move outwards and inwards. So how is it that the energy of the system increases in one case while decreases in the other? Which forces perform some positive work so as to cause an increase in the energy of the system?