What happens when a spinning skater pulls her arms in?
After you watchWhat happens when a spinning skater pulls her arms in?
The short answer
When a spinning skater pulls her arms in, she spins faster because her total spin-amount (angular momentum) stays the same, but it is now packed into a much smaller circle — so she has to whirl faster to carry it all.
Try this next
- Does the same thing happen when a diver tucks during a somersault? Watch a diving video and notice how fast the flip gets when the diver pulls into a tight tuck.
- What if you added weight to the skater's outstretched hands? Think about whether heavier hands at the end of the arms would make pulling them in even more dramatic.
- Does a spinning top slow down for the same reason? Notice that a top's shape never changes — so it slows down for a different reason (friction). What's the difference?
The whole story
How it works
Every spinning object carries a fixed quantity called angular momentum (L = I × ω). When the skater pulls her arms in, her moment of inertia (I) — a measure of how spread-out her mass is — gets smaller. Because L must stay the same and L = I × ω, a smaller I forces ω (her spin rate) to increase. No push is added; the spin-amount is conserved.
What people get wrong
Many people think the skater must push off the ice or kick to spin faster. In fact she adds no external force — she simply rearranges her own mass by pulling her arms in, which automatically increases her rotation rate through conservation of angular momentum.
The catch
Arms wide means a graceful, controlled, slower spin — easier on her muscles. Arms in means a dazzling, blazing-fast spin — but it is exhausting on her core and difficult to control at that speed.
Questions kids ask
Does she use more energy when she spins faster?
Yes — her kinetic energy increases when she pulls her arms in. The extra energy comes from the work her muscles do pulling her arms inward against the inertia of her own mass, which tends to keep moving outward in a straight line.
Why doesn't the spin-amount increase too?
Angular momentum only changes if an outside force (torque) acts on the skater. On nearly frictionless ice with no external push, there is essentially no external torque, so the spin-amount is conserved.
What happens when she stretches her arms back out?
The reverse: her moment of inertia increases, so her spin rate slows back down — the spin-amount stays the same throughout.
Does this work on a spinning chair at home?
Yes! Sit on a swivel chair, spin yourself, then quickly pull your arms and legs in. You will feel yourself spin faster — the same physics as the skater.
Talk about it
- Have you ever spun on a swivel chair and pulled your arms in — what did you feel?
- Why do helicopters have a tail rotor, and how does the main rotor affect the body of the helicopter?
- Can you think of other sports where athletes pull their limbs in to spin faster?
For grown-ups
Conservation of angular momentum: L = Iω stays constant when no external torque acts on the skater. With arms extended, the moment of inertia I is large and angular velocity ω is small. When she pulls her arms in, I decreases dramatically (mass closer to the rotation axis), so ω must increase proportionally. Note that her kinetic energy (½Iω²) actually increases — the extra energy comes from the internal work her muscles do pulling the arms inward.
Keep going
What else makes you wonder?
- What would happen if a figure skater could stretch her arms out even farther than normal — would she slow down even more?
- Why do divers tuck their body into a ball while flipping — is the same idea at work?
- Could you feel the spin-up on a spinning chair if you pulled your arms in?