Arms In, Spin Up — One Number Refuses to Move

You'll learnwhy a skater's angular momentum stays fixed while her rotational kinetic energy changes.

A skater spins slowly on frictionless ice, arms flung wide. She hauls them in and whirls faster — no push, no engine. Her angular momentum never budges, yet her kinetic energy triples. Two ledgers, two verdicts.

8 STEPS · 6 QUICK CHECKS · v2

ROTATION LAB

Before you start

What you're looking at

A skater spinning slowly on frictionless ice with arms flung wide, beside two ledger bars for L and K.

The question

When she pulls her arms in with no outside push, what happens to her spin and her energy?

Watch for

The pink L bar staying pinned while the green K bar climbs as she pulls her arms in.

What the symbols mean

The pink bar is angular momentum L = Iω; the green bar is rotational kinetic energy K (joules).

Step 1 / 8