i About this experiment — click to learn the physics ▼
What you're looking at
A platform spins freely with two weights held out on arms — like a figure skater, or someone on a rotating stool holding dumbbells. With no outside twisting force (torque), the spinning system's angular momentum is conserved. Slide the arms in and out and watch the spin rate respond instantly.
Angular momentum and inertia
Angular momentum is the rotational equivalent of ordinary momentum:
The moment of inertia I measures how hard the mass is to spin, and it depends strongly on how far the mass sits from the axis — each weight contributes m·r². Holding the weights far out gives a large I; pulling them in shrinks it dramatically (the radius is squared).
Why pulling in speeds you up
Because L can't change, if I goes down then ω must go up to compensate:
Halve the arm length and each weight's r² drops to a quarter, so I plummets and the platform whirls much faster — exactly how a skater accelerates a spin by drawing arms and legs to the body.
Where does the extra energy come from?
Watch the rotational KE climb as you pull in, even though L holds steady — since KE = ½·L²/I, a smaller I means more energy. That energy isn't free: you do work pulling the weights inward against their tendency to fly outward. Let them back out and that energy is returned. Angular momentum is conserved; kinetic energy is not.
Things to try
Spin it up, then pull the arms all the way in and watch ω rocket up while the gold L readout never budges. Use the slow-motion button to follow the fast spin. Then push the weights back out and see the spin slow to its original rate.