## Answer #85

The answer is (c): the bicycle wheel gyroscope will rotate clockwise
around the pivot, a motion known as "precession," as seen in an mpeg
video by clicking your mouse on the photograph below.

When Gwen spins the gyroscope, its rotation can be represented by a
vector as follows: curl the fingers of your right hand in the direction of
rotation of the wheel, and your thumb will point in the direction of the
angular momentum vector. This is in the direction *away* from
Gwen. The torque, created by the gravitational force on the wheel, is
given by the cross product of the vector distance from the pivot to the
center of mass of the wheel **r** and the vector force
**F**, or **rxF**, which is toward the right front of the
photograph from where Gwen is positioned. This causes the wheel to
precess around the pivot in the clockwise direction, as seen.

Note also the slight scalloping motion of the wheel as it
precesses. This motion, called "nutation," occurs because the gyroscope
begins to fall downward immediately after it is released but before its
precession ensues.

Archive 5
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