Why Skaters Spin Faster with Their Arms In

An ice skater begins a spin with both arms stretched out. Then the arms move towards the chest, and the spin accelerates. The skater has not pushed against the ice again. So where does the extra speed come from?

The answer lies in how the body's mass is distributed around its axis of rotation—the imaginary line around which it turns. During an upright spin, this line runs through the skater from head to foot. With arms extended, more of the body's mass is far from the axis. Pulling the arms in brings that mass closer.

Mass farther from the axis makes a rotating object harder to speed up or slow down. This resistance to changes in turning motion is called its moment of inertia. Pulling the arms inward reduces it: the skater's total mass remains the same, but its distribution changes.

Another important concept is angular momentum, a measure of turning motion. When there is little external twisting force, angular momentum remains nearly constant. As the skater's moment of inertia decreases, the spin therefore becomes faster. Extending the arms again has the opposite effect: the spin slows.

This does not mean that the increase in speed requires no energy. Pulling the arms inward during a spin takes muscular effort, which adds energy to the turning motion even while angular momentum remains nearly unchanged. Air resistance and friction also prevent a real spin from continuing forever.

Divers use the same principle in the air. A body with its arms and legs tucked tightly in can turn faster than an extended one. Opening the body slows the rotation before entry into the water.

These movements show why sporting skill is not simply a matter of strength. Athletes also control how their mass is distributed. A small change in body shape can produce a large, visible change in movement.