I Torque required to prevent precession

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    Precession Torque
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A torque is applied to a spinning gyro to prevent precession, specifically when the gyro is constrained to rotate only about the x-axis. The discussion highlights the need to quantify the torque exerted by the gyro about the y-axis and the corresponding reaction from the frame to maintain stability. It is clarified that the gyro is not affected by gravity, which simplifies the analysis of angular momentum changes. The conversation touches on the implications of blocking rotation about the y-axis, suggesting that the gyro would behave as if it had no angular momentum and would drop. Overall, the mechanics of torque and angular momentum in this constrained system are central to understanding gyro behavior.
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A torque is applied to a spinning gyro, and a second torque is applied to prevent precession. How to quantify the second torque?
A torque is applied to a spinning gyro, and a second torque is applied to prevent precession. How to quantify the second torque?

Example, a gyro spinning about the z axis is connected to a frame that can only rotate about the x axis. A torque about the x-axis is applied to the frame. What is the torque about the y-axis exerted onto the frame by the gyro and its Newton third law counterpart, the torque the frame exerts onto the gyro about the y axis, that prevents the gyro from precessing? The rate of change in angular momentum about the x-axis would be due to the combined external torques.
 
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rcgldr said:
Summary: A torque is applied to a spinning gyro, and a second torque is applied to prevent precession. How to quantify the second torque?

A torque is applied to a spinning gyro, and a second torque is applied to prevent precession. How to quantify the second torque?

Example, a gyro spinning about the z axis is connected to a frame that can only rotate about the x axis. A torque about the x-axis is applied to the frame. What is the torque about the y-axis exerted onto the frame by the gyro and its Newton third law counterpart, the torque the frame exerts onto the gyro about the y axis, that prevents the gyro from precessing? The rate of change in angular momentum about the x-axis would be due to the combined external torques.

Are we to assume that this gyro is not affected by gravity?

Since there is a constant torque about the x axis, the torque vector is a constant vector along the x axis. As a result, if the frame was free to move about all axes, the change in the angular momentum vector would be: ##d\vec{L}=\vec{\tau}dt## ie the change in angular momentum is in the direction of the x axis.

The resultant of ##\vec{L}+d\vec{L}## would be a new ##\vec{L'}## in which the axis of spin has rotated about the y-axis at an angle to the z axis ##\alpha## such that ##\alpha=\frac{\vec{\tau}dt}{\vec{L}}##.

But that cannot occur, because rotation about the y-axis is not allowed. So it seems to me that the frame must apply a torque to the x-axis that is equal and opposite to the applied torque along the x-axis to negate that applied torque.

AM
 
Andrew Mason said:
Are we to assume that this gyro is not affected by gravity?

But that cannot occur, because rotation about the y-axis is not allowed. So it seems to me that the frame must apply a torque to the x-axis that is equal and opposite to the applied torque along the x-axis to negate that applied torque.
The gyro in question is not affected by gravity.

The frame is free to rotate about the x-axis, so how does it generate a torque about the x-axis?

I recall some video where with the same or similar configuration: if rotation about y-axis is blocked, the reaction is the gyro accelerates about the x-axis as if the gyro was not spinning, no angular momentum, just angular inertia.

I haven't been able to find that video again, but I did find this old (1974) example claiming that there is no angular momentum related to precession. I set the link time to where an 8 lb gyro supported at one end is spun up to several thousand rpm and is precessing (with assist to get it near level) due to torque from gravity, then a small peg is placed in a hole that blocks the other end, stopping the precession, and the gyro just drops down (and bounces back a bit). The support structure is then vertically oriented, but there's too much play in the hole, so the peg moves instead of breaking. In another part of the video, a false claim is made about assisting the precession affects the downwards force at the support, but any movement in the supporting spring is probably nutation.

 
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