Science Puzzle
The Gyroscopic Wheel
A bicycle wheel is spinning fast. You hold one end of its axle with a rope attached to the ceiling and let go of the other end. Instead of falling, the free end of the axle traces a slow horizontal circle. The wheel stays up.
Why does the spinning wheel not fall?
The Answer
Gyroscopic precession. The spinning wheel has angular momentum directed along its axle. Gravity exerts a torque on the wheel, but that torque is perpendicular to the angular momentum vector, not against it.
According to Newton's laws in rotational form, a torque changes the direction of angular momentum, not its magnitude. So the torque from gravity does not slow the spin or pull the wheel downward; it rotates the angular momentum vector horizontally. The axle swings in a slow circle instead of falling.
The faster the wheel spins, the greater its angular momentum, and the slower and more stable the precession. A very slowly spinning wheel precesses quickly and wobbles; a very fast one precesses slowly and stays level.
This same principle stabilises gyrocompasses, inertial navigation systems, and the flight of spinning bullets and frisbees.
The principle: Gyroscopic precession. A spinning object has angular momentum. An applied torque rotates the angular momentum vector rather than opposing it. The result is precession: rotation perpendicular to the applied force.