Slide 1
A skater spins faster by drawing the arms inward
A spinning skater can rotate faster by drawing their arms inward. The effect depends on how mass is distributed around the spin axis.
More to Read
Rotational inertia measures resistance to a change in rotation and depends on distance from the axis. Moving the same mass farther outward increases its contribution strongly, even though the total mass stays unchanged.
Slide 2
Angular momentum tracks how mass and rotation combine
For rotation about a fixed axis, angular momentum combines angular speed with rotational inertia: resistance to a change in spin.
It stays constant without net external torque, a turning effect from outside.
More to Read
Torque is the turning effect of a force about an axis. A push passing through the axis has no lever arm about it, while an offset push can change the angular momentum.
Slide 3
Moving mass inward reduces resistance to changing the spin
Pulling mass closer to the axis reduces rotational inertia. With angular momentum approximately conserved, angular speed must increase.
More to Read
If rotational inertia becomes half its original value, angular speed doubles in the ideal isolated-axis model. The relationship concerns turns per time, not the unchanged total mass of the skater.
Slide 4
The muscles supply the extra rotational energy
Faster spinning can mean greater rotational kinetic energy. The skater’s muscles do work while pulling the arms inward.
More to Read
Conserving angular momentum does not require conserving rotational kinetic energy by itself. Internal chemical energy can become mechanical energy while there is little external torque, so the two statements are compatible.
Slide 5
Friction and air resistance limit the approximation
Friction and air resistance exert external torques that gradually slow rotation. Conservation gives an approximation over a suitably short interval.
More to Read
Extending the arms reverses the redistribution and reduces angular speed. It also changes exposure to air, so a real performance combines the clean conservation effect with additional dissipative influences.
Sources
- Conservation of Angular Momentum
The Physics Classroom
- STEMonstrations: Kinetic and Potential Energy
NASA