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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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