The Bicycle Wheel Gyroscope


The Demo: The man is standing on a rotation platform. When he spins the bicycle wheel and turns it sideways, he also spins around. What's going on?

Quick Physics
: Conservation of Angular Momentum means that the man turns in the opposite direction from the spinning bicycle wheel.

The Details:

This illustrates an important conservation law of physics: the conservation of angular momentum or turning motion. Anything that is turning has angular momentum. This is similar to inertia, only in rotational motion. If a wheel is spinning in one direction, it wants to keep turning in that same direction. If it is not turning, it tries to stay still.

To show this, someone stands on a level platform that can spin around. If she gets on carefully, so that she’s not turning, no matter now much she twists and dances, she won’t be able to spin all the way around. Next, someone else can push her so she starts spinning. Now, no matter how hard she tries, she won’t be able to stop spinning. This is conservation of angular momentum.

In the bicycle wheel gyroscope demonstration, there are two possible turning motions. The platform the person stands on turns like a wheel on its side. Call this horizontal turning motion. The other turning motion is the bicycle wheel the person is holding. Initially it is vertical, as a bicycle wheel normally stands. Call this vertical turning motion.

To begin with, someone spins the bicycle wheel. There is vertical turning motion in the bicycle wheel, but no horizontal turning motion of the platform. Then the person tilts the bicycle wheel to the side, so now there is horizontal turning motion where there wasn’t any before. Conservation of angular momentum tries to fix this. The platform starts spinning in the direction opposite that of the bicycle wheel. If you add the backward turning of the platform and the forward turning of the bike wheel together, you get zero. So horizontal motion is the same as when we started: nothing. When the bike wheel is tilted back vertically, the platform stops spinning, as in the beginning.

Rocket ships are stabilized this way. They have little wheels spinning inside called a gyroscope. When the rocket ship turns, the spinning wheels turn the other way and bring the rocket back on course. Football players also put a spin on the football to improve the accuracy of their passes.