A wheel is set spinning and then is hung by a rope placed at one end of the axle.The precession vector of the spinning wheel points in the direction of
A)z
B)-y
C)-z
D)-x

Question

A wheel is set spinning and then is hung by a rope placed at one end of the axle.The precession vector of the spinning wheel points in the direction of 
A)z 
B)-y 
C)-z 
D)-x

Quizplus · Accepted Answer

The precession vector points in the direction of the torque applied to the spinning wheel. In this case, the torque is due to the force of gravity acting on the center of mass of the wheel. Since the wheel is hanging from the rope, the force of gravity acts in the -z direction (downward). According to the right-hand rule, the direction of the precession vector is perpendicular to both the applied torque and the angular momentum vector. Therefore, the precession vector points in the direction of the -y axis. However, we are asked for the direction of the precession vector in terms of the fixed coordinate system. The precession vector rotates around the vertical (z) axis with a constant frequency, so we can think of it as a vector with an angular velocity in the -x direction. Therefore, the correct answer is C) -z.

A Wheel Is Set Spinning and Then Is Hung by a Rope