Science Puzzle
Why Does the Spinning Skater Speed Up?
An ice skater spins with her arms stretched out wide. Then she pulls her arms in tight against her body, and without pushing off the ice at all, she suddenly spins much faster.
Where does the extra speed come from?
The Answer
Her total amount of spin cannot change on its own, so when she moves her mass inward, the rotation must speed up to compensate. Physicists call the conserved quantity angular momentum, and it depends on both how fast you turn and how far your mass sits from the centre.
With her arms flung wide, a lot of her mass, her hands and arms, sits far from the spin axis, so she can turn slowly and still have plenty of spin. Pull that mass in close to the axis and, to keep the same total spin, the whole body has to whirl round faster. She did not add any energy from the ice; she just rearranged where her mass was, and the physics did the rest.
Divers and gymnasts use the identical trick: tuck to spin fast, open out to slow down before landing.
The principle: Conservation of angular momentum. When no outside twist acts on a spinning body, moving mass closer to the axis increases the rate of spin.