# Free Pivot of a Gyroscope

1. Oct 30, 2008

### breez

Hey, while reading Kleppner & Kolenkow's discourse on gyrocompasses, I ran into a statement I could not wrap my head around:

"The pivots at A and B allow the system to swing freely about the y axis, so there can be no torque along the y-axis."

AB is an axis along the y-axis, which is the axis along which the gyrocompass can freely swing. The spin axis is along the x-axis.

How can this be true? If the gyrocompass can swing along the y-axis from initially no angular velocity along the y-axis, it must have some torque to accelerate it, and hence torque along the y-axis. This seems extremely paradoxical to me. Can someone explain?

2. Oct 31, 2008

### pallidin

Not sure if this helps, but I found this:

"A gyrocompass is essentially a gyroscope, a spinning wheel mounted on gimbals so that the wheel's axis is free to orient itself in any way. Suppose it is spin up with its axis pointing in some direction other than the celestial pole. Because of the law of conservation of angular momentum, such a wheel will maintain its original orientation. Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is rotating once every 24 hours. Such a rotating gyroscope cannot be used for navigation. The crucial additional ingredient needed for a gyrocompass is some mechanism that results in applied torque whenever the compass's axis is not pointing north..."

Source: http://en.wikipedia.org/wiki/Gyrocompass