Small merry-go-round

What can you do here
You can stand or sit on the merry-go-round. Ask for weights. Hold them close to your torso. Now ask to be spun around. Try holding the weights away from you or pulling them close, keeping them for a moment in either of the extreme positions. Note how fast you are spinning while doing that.

How does it work
You’re spinning more rapidly with your hands close to your body than with hands outstretched. It happens due to the law of conservation of angular momentum in a physical system. The system is composed of you and the platform. The angular momentum is a notion related to the mass, the size, and the rotational speed of the system. Since the mass remains constant, when the system size increases, its rotation speed decreases – and vice versa, a smaller size means a larger rotation speed (called the angular velocity).

Interesting facts
The effect under discussion is used by pirouetting dancers. When pulling their hands towards themselves they spin faster. It is also not unknown to cats, which always fall on all four legs. A cat first pulls its forepaws in and turns the front part of its body. It causes its posterior part to turn in the other direction but, as its back paws are stretched, the turn isn’t significant. Then, the cat puts outs its forepaws, pulls the back paws closer and turns the other half of its body into the right position.