Gyroscopes
Gyroscopes are great fun. Sometimes the labels or advertising for the boast such claims as
DEFIES GRAVITY!
Gyroscopes may, indeed, be amazing. But do they really defy gravity? How do they work?
First, consider a gyroscope that is not spinning. Look at the forces and torques on it when it is supported at one end and held (and then released) from a horizontal position (ie, with the axis is a horizontal position).
Remember
And, as the non-rotating gyroscope falls over, its change in angular momentum does, indeed, line up with the torque.
Nothing unusual happens with a non-spinning gyroscope.
However, look at a spinning gyroscope. Consider one spinning fast enough that its angular momentum vector L always lines up with its axis of rotation.
The forces -- and torques -- are exactly the same as they were before.
Remember
The change in the angular momentum vector must point into the screen. That means the gyroscope must precess so the right end of its axis moves into the screen.
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