Bicycle Wheel Gyro at the Exploratorium

(http://www.exploratorium.edu/snacks/bicycle-wheel-gyro)

The exploratorium in San Francisco, CA is full of a wide variety of science experiments that exemplify various scientific principles. The many stations and activites make it hard to choose just one to discuss. Near the entrance of the museum is an exhibit consisting of a low-friction rotating stool and a bicycle wheel with the ability to spin on its axle.

Handles are fixed onto each side of the wheel's axle, allowing the user to hold the handles while the wheel spins freely. The activity takes two people to work. One person sits in the chair, holding the wheel, while the other starts the wheel spinning. Next, the person on the stool lifts their feet off of the stool while tilting the wheel. If done correctly, the person on the stool should start to spin.

The explanation of this exhibit has to do with the angular momentum of the wheel, which involves the speed of rotation, the mass of the wheel, and how the wheel's mass is distributed. On the bicycle wheel, the mass is concentrated along the rim of the wheel, allowing for a larger angluar momentum at a given speed.

The bicycle wheel, stool, and person form a system which abides by the principle of conservation of angular momentum. This means that change in the system must be countered with an equal and opposite change, resulting in a net effect of zero.

The angular momentum is characterized by both size and direction, so when the direction of the wheel is changed, the angular momentum is also altered. When the direction is changed, the person in the stool exerts torque, a twisting force, on the wheel. In response, the wheel exerts an equal and opposite force of torque on the person. Thus, when the bicycle wheel is twisted in space, the person will twist the opposite way on the low-friction stool.

A vital aspecta of this activity is the speed of the spinning wheel. The wheel must be spun fast enough so that the torque experted on the person is enough to create noticible movement. Also, the wheel must be kept at a high speed through out the activity. Due to friction, the wheel's velocity will gradually decrease, so the starting speed must be bery high to compensate for this inevitable decrease.

Abstractly, many students understand torque and equal, opposite forces, but having a physical demostration often helps students to apply concepts to a real-life situation. Having the ability to understand how different forces and significant concepts interact can help students solve problems inside and outside of the textbook.

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