# Precession Torque

The spin angular momentum is along the rotation axis as shown, but the torque about the support point is in a direction perpendicular to the angular momentum. The torque produces a change in L which is perpendicular to L. Such a change causes a change in direction of L as shown but not a change in its size. This circular motion is called precession.

If the wheel is not spinning, the apparatus just rotates downward toward a vertical orientation. If you curl the fingers of your right hand in the direction of that expected rotation, then your thumb will point perpendicular to the spin axis in the direction of the torque produced by gravity. The tip of the angular momentum vector, and therefore the axle of the wheel, will precess in that direction.

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# Precession, Top View

The spin angular momentum is along the rotation axis as shown, but the torque about the support point is in a direction perpendicular to the angular momentum. The torque produces a change in L which is perpendicular to L. Such a change causes a change in direction of L as shown but not a change in its size. This circular motion is called precession.

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Go to side view

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