Play Ground Swing

The com rises at the bottom, decreasing the effective radius. Positive work is being done by the centripetal force, hence generating kinetic energy. The straightening action is done at the bottom of the arc where the centripetal force is maximum, whereas the squatting action is done at the high points where the centripetal force is minimum. As a result positive work is done and hence net kinetic energy is pumped into the system.  

In the second case, with the person sitting on a swing, a very different mechanism is at work.  Work is being done at the two high points where a torque is exerted to change the orientation of the body. No action is exerted at the bottom of the arc. In the first case kinetic energy is added at the bottom of the arc and in the second case potential energy added at the high points.

mandrin