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Q & A: Spinning Around

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Most recent answer: 10/22/2007
Q:
What can I do for a bicycle wheel demonstration?
- Tonya Jernigan
Abilene, TX
A:
Many cool bicycle wheel demonstrations are based on something called "conservation of angular momentum". Conservation of angular momentum means that when something is spinning, it will tend to keep spinning that same way. The key to the demonstration is that direction is important. Holding a spinning wheel with its rotation axis pointing up is different from holding the same spinning wheel with its rotation axis pointing sideways.

Since direction is important, we can do some really cool demonstrations with it. Sit on a stool that can rotate. Take the bicycle wheel and start it spinning. If you tilt the wheel, you change its angular momentum. Since angular momentum is not supposed to change, the stool will spin in order to keep the overall momentum the same.

The other really neat demonstration is harder to do. You need a rope hanging from the ceiling with a ring on the bottom. Or you can use a pole on the ground with a flat top. The wheel gets spun really fast and put up & down with one end on the ring. You would think the wheel would fall off, but it doesn't! It moves slowly around the ring. This is due to angular momentum again. When it tries to fall, the angular momentum will have to change. Since it doesn't like to change, the wheel will move in a circle without falling. You have probably seen a child’s toy called a “Gyroscope”; its exactly the same thing.

Adam

(published on 10/22/2007)

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