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Gyroscope Precession
Name: Mike D.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 8/26/2004
Question:
Is the right-hand rule for angular momentum of a
gyroscope dependent on the hemisphere in which the gyroscope is
spinning? If not, why does the gyroscope have angular momentum in one
direction with no other variables influencing it?
To clarify, when I have a bike wheel on a peg, vertically, in front of me
and I spin it clockwise from the perspective of someone to the right of
me, why does the wheel turn to my left?
In space, does a gyroscope act this same way? With no other variables to
influence the gyroscope, why would it turn one way and not the other?
Replies:
Mike D.,
The direction of precession depends on the direction the wheel is spinning,
not the hemisphere.
Angular momentum and torque work together in the same way as linear momentum
and force. Force, a linear push, causes momentum to change. Torque, a
twisting push, causes angular momentum to change. Also, angular momentum
points along the axis of rotation. Torque points along the axis about which
it twists.
In some ways, this gyroscope effect is similar to the force and momentum of
a rock on a string. The rock spins in a circle. Momentum is tangent to the
circle. Force due to the string is toward the center, perpendicular to the
circle. The momentum keeps rotating in the direction of force, but force
keeps rotating as well. All momentum can do is keep rotating.
Consider a gyroscope on its side, pointing toward you. You see the
gyroscope as spinning counter-clockwise. The angular momentum vector points
right at you. Gravity tries to make the top of the gyroscope move downward.
The torque must then be perpendicular to the gyroscope axis. Torque due to
gravity points to the right. The angular momentum is then pulled a little
to the right by this torque. As the gyroscope rotates, so does the torque
axis. The angular momentum never gets a chance to line up with the torque.
All the angular momentum can do is continuously rotate toward the direction
of the torque.
Gyroscopes would do nothing in outer space. With no gravity to exert the
torque, there would be no reason for angular momentum to change direction.
The spinning gyroscope would not turn.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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Update: June 2012
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