Spinning demo

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While spinning, an ice skater can control the speed of rotation by moving his or her arms. With arms stretched out, the skater's rotation is slower that with arms pulled in close to the body. This phenomenon can be understood by applying the law of conservation of angular momentum.

Below are two figures of a physics student. In one figure she is holding weights at a distance from her body. It the second figure, the weights are held close to her body. In this second figure she has decreased her moment of inertia compared to the first figure since the weights are at a smaller radius from her body.

While spinning, angular momentum is conserved. This means that the value of the angular momentum does not change. If angular momentum is expressed at L=I, one can see that if the moment of inertia increases (I) then the rotational frequency must decrease in order to keep L constant. This principle is demonstrated in the movie.

1. If a skater speeds up, there is an increase in kinetic energy. Where does this increase come from?
2. When a diver is doing flips and wants to twist, he or she extends only one arm, not two. Why does this make the diver twist?