MadSci Network: Physics

Re: Will the speed of a perfect gyroscope decay if forced against precession

Date: Mon Feb 19 10:26:39 2001
Posted By: Juan Cabanela, Faculty, Physics, Astronomy, & Engineering Science, Saint Cloud State University
Area of science: Physics
ID: 980725729.Ph

Well, the problem, as it has been set up, is an impossible situation, 
and let me explain why.  If you turn the fork (let us say you turn it 
counter-clockwise as viewed from above), the torque on the spinning 
wheel will be such as to drive the wheel to move its angular 
momentum vector upward.  This motion is clearly not being allowed 
since by the setup of the problem, we are insisting that the wheel be 
locked into the fork by a frictionless axle and furthermore the frame of 
the bike is not allowed to accept any of the angular momentum.  
Therefore, there must be an equal and opposite torque, provided by a 
normal force from the bike frame, such that the angular momentum 
is constant (since you are not allowing the wheel to transfer its 
angular momentum elsewhere).  Therefore, according to the setup of 
the problem, there is no way to couple the angular momentum of the 
spinning bike tire to any other object, and therefore, by conservation 
of angular momentum, the bike tire can not change the direction of its 

It should be made clear that there is no "natural precession" of a bike 
tire in the absence of outside forces acting on the bike tire.

So why can you turn a bike tire on the ground?  Because in that case, 
the bike tire's angular momentum is coupled both to the frame of the 
bike and to the Earth.  Therefore, angular momentum can be 
transferred to other items and the bike tire can turn.

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