1Two things to know
Spin-amount and spread-out
Every spinning skater carries a "spin-amount" that stays locked in — and how spread-out she is decides how fast she uses it.
Spin-amount stays the same
Once she's spinning, nobody pushes her. The total spin-amount stays exactly the same the whole time — it's locked in like a sealed bottle of spin.
How spread out she is matters
The same spin-amount can live in a big circle or a small one. But that changes something about the spin. The question is — what?
2The two positions
Arms wide vs arms tight
Both positions use the exact same spin-amount — only the spread is different. Which one spins faster?
Spin-amount spread wide
Her spin-amount is spread across a big circle. What does that mean for her speed?
Spin-amount packed tight
Same spin-amount — but now packed into a small circle. Does that change the speed?
3Your turn — be the coach
Move her arms and watch her shape change
Slide her arms from wide to tight. Notice how her shape changes — but don't worry yet about what happens to her speed. That's the experiment.
4The experiment
Predict before you watch
A skater is already spinning with her arms stretched wide. She pulls her arms all the way in — but nobody pushes her. What happens?
Guess before you find out
She pulls her arms in — but doesn't push or kick. Does she spin faster, slower, or stay the same?
Step by step · calm telling
5Nothing is free
Each position has a cost
She spins slowly and smoothly. Easy on her muscles, and she can hold the pose for a long time.
She whirls at dazzling speed — the crowd gasps. That's the Olympic moment.
When she pulls her arms in, she packs the same spin-amount into a smaller circle — so the spin gets faster. No push. Just physics.
Psst, grown-ups: this is conservation of angular momentum: L = Iω stays constant when no external torque acts. I is the moment of inertia (how spread-out the mass is — larger when arms are extended). ω is the angular velocity (spin rate). When the skater pulls her arms in, I decreases, so ω must increase to keep L the same. Note: kinetic energy (½Iω²) actually increases — the extra energy comes from the work her muscles do pulling the arms in.