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MIT Physics Demos: Conservation of Angular Momentum

3 minutes

(Describer) A man stands on a turntable and holds a spinning wheel.

When I hold the spinning wheel,

(Describer) ...sideways.

nothing crazy happens. But when I flip it, I spin,

(Describer) He does.

(Describer) ...on the turntable.

But back... and I stop. Flip it, I spin.

(Describer) He does it again.

So the question is, "Why am I spinning?"

(Describer) He spins clockwise, the wheel counter-clockwise.

I have to force the wheel to turn over.

(Describer) ...to clockwise.

Newton's third law says for every force, there's an equal and opposite force. So when I force the wheel,

(Describer) ...flipping it.

the wheel is applying force equal and opposite onto me, causing me to turn.

(Describer) He gets off the turntable.

The same applies when I'm on the ground, but friction between my shoes and the ground stops me from spinning. Another way to explain what's going on is through... Now, angular momentum tells you how hard it is to either start or stop something from spinning and depends on two things. One: How fast it's spinning. We call that the... Two: Its shape, or what we call the... So this wheel is rotating counterclockwise. Here's a trick. Get your right hand and curl your fingers in a counterclockwise direction. You see that your thumb is pointing upwards. We'll call that the positive direction. So, see, this wheel has positive angular momentum with a magnitude of L. Angular momentum is conserved. As long as there are no outside forces, the total angular momentum of the system stays the same. When you flip this over and do the right-hand trick, your thumb points down or in the negative direction. The angular momentum of this wheel is now negative L. So the whole system--me, the turntable, and the wheel-- starts out at positive L. But when you flip the wheel, the wheel now has negative L.

(Describer) ...and he turns...

The whole system needs to stay at positive L because of conservation of angular momentum. So I need to spin with angular momentum of positive 2L. That's why I spin the other way.

(Describer) Title: Made with love at MIT. Accessibility provided by the U.S. Department of Education.

Accessibility provided by the U.S. Department of Education.

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When Ryan spins a bike wheel, nothing crazy happens. But something weird happens when he flips it over. He explains the physics behind the conservation of angular momentum. Part of the "MIT Physics Demos" series.

Media Details

Runtime: 3 minutes

MIT Physics Demos
Episode 1
3 minutes
Grade Level: 9 - 12
MIT Physics Demos
Episode 2
3 minutes
Grade Level: 10 - 12