Conservation of angular momentum

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In this demonstration, a student holds a spinning bicycle wheel while standing on a platform that is free to rotate. The student is not rotating at first, but begins to do so when the orientation of the bicycle wheel changes. Why does the student start to rotate?

This demonstration is one of the most popular and fun to do, yet its full explanation is quite complicated. However, it is useful to demonstrate the basic principle of conservation of angular momentum.

In the figures above, the angular momentum vectors of the person-tire system are shown. When the person turns the tire upside down, the angular momentum vector of the tire changes sign. The force used to flip the tire is an internal force, so angular momentum is conserved (remains constant). In order for the final total angular momentum to equal the initial angular momentum, the system starts rotating. This fun demonstration can be seen in the movie.

1. If the student begins to rotate, there is an increase in kinetic energy. Where does this increase come from?
2. If the tire is spun in the opposite direction before it is flipped, which way will the person rotate? Why?