The Arm Engine

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If you've ever watched a figure skater twirl, you know what happens when she pulls in her arms toward her body: what begins as a slow, graceful spin gets faster and faster, thanks to a principle of physics known as angular momentum conservation. Add hand weights, as we have in this dizzying experiment, and the effect is even more dramatic.


• 2 people
• Swivel chair
• Set of dumbbells or arm weights or 2 16-ounce or larger cans of food


1. Have one person sit in the chair holding the dumbbells or cans with her arms outstretched.
2. Have the other person set the chair spinning.
3. Ask the sitter to bring her arms into her chest. The chair will start to spin noticeably faster.


Angular momentum is a measure of how hard it is to stop something that's spinning. The heavier, wider, and faster-spinning an object, the more difficult it is. If you make a spinning object narrower, it spins faster because its angular momentum must stay the same. That's why the girl on the chair speeds up when she makes herself narrower by pulling in her arms. The weights accentuate the change in shape and thus make the speedup more dramatic.


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