Spin

pull your arms in. the universe keeps the books.

drag a skater’s arms in and out — spin rate obeys a conserved law

Angular momentum — spin rate times how spread out the mass is — can’t change on its own. Pull the arms in and the spin has to speed up to keep that product constant, with no extra push added anywhere; the increase is the conservation law paying out, not added force. Collapsing stars spin up the same way.

Concepts: angular-momentum, physics.

An interactive you play with one thumb, on Wundr.