# Understanding gyroscopes

Considering the typical situation of a rotating bicycle wheel held by one end of its axle by a rope tied to the ceiling: gravity torque is the time derivative of the angular momentum, and in this case is perpendicular to the angular momentum vector which makes it revolve without changing its magnitude (it precesses).

My question is the following: the wheel stops turning eventually because of the friction in the string (it also slows down its own rotation because of friction in the bearing but let's ignore it), but could you clarify how, step by step? Is it because the gyroscope creates a torque on the string axis, but it's reduced by the friction torque, and as a result the friction torque translates back into a residual "gravity torque" (in other terms, the gyroscope "gives up" on a certain amount of gravity torque it can't counteract because of the string)?

To progress toward my actual problem in several dimensions, what if the friction torque is in fact torque given by another gyroscope on the same solid?

• I am having a hard time figuring out what it is you are trying to describe, could you upload an image of the system? Also, what are "gravity torque" and "friction torque," I have never heard of such things. Commented Feb 26, 2014 at 18:12
• Thanks for your answer. I updated the description with a link to one of many videos that take pretty much the same example everytime. As for the torques, I should probably have been more precise in saying those are torques generated by the weight of the wheel on the one side and by the friction in the torsion of the string on the other. Commented Feb 26, 2014 at 22:43
• As usual there is much to be gained by reading 'It Has to Go Down A Little, In Order to Go Around'- Following Feynman on the Gyroscope. The short-short version is that our usual classroom explanation is incomplete, and shows that a slightly more complete version reproduces much more of the dynamics. Commented Jul 17, 2014 at 14:14