### Small merry-go-round

What you can do here
Depending on the model, you can stand or sit on the merry-go-round. Ask for weights. Hold them close to your body. Now ask someone to spin you 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 spin while doing that.

How it works
You spin more quickly with your arms close to your body than with your arms outstretched. This is due to the law of conservation of angular momentum in a physical system. The angular momentum is a notion related to the mass, the size (the mass and the size express the so-called moment of inertia), 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 is used by pirouetting dancers. When they pull their arms towards their body, they spin faster. It is also known to cats which always fall their four legs. A cat first pulls its front paws in and turns the front part of its body. This makes its posterior part turn in the other direction but as its back paws are stretched, the turn isn’t big. Then, the cat puts out its front paws, pulls the back paws closer and turns the other half of its body into the right position. You can do a similar experiment in Racing on an inclined plane.