What is the main cause of the Gyroscope Torque Confusion?

[DocZaius]

This is not a homework question, but from having read the rules in the following thread, it looks like this is more academic assistance than pure discussion:

https://www.physicsforums.com/showthread.php?t=414380

This should be done whether the problem is part of one's assigned coursework or just independent study. The reason for this is that the scientific and mathematical sections of Physics Forums are to be reserved for discussions and not academic assistance.

So anyhow, I am having real trouble with precession. I know that net torque is equal to the derivative of angular momentum with respect to time.

If you put a weight at a distance from the axis of a non-spinning wheel, it tilts down. If you do the same at a distance of a spinning wheel, it rotates at a constant angular velocity. Attached is a picture trying to illustrate my confusion.

I just can't understand why the same torque with the same dL/dt results in completely different looking phenomena. Of course I know that the initial conditions are different in each case. But I don't understand what it is about their differences that leads to any accelerating tilting gyroscope to a constantly spinning gyroscope. Any help would be appreciated. The Wikipedia page did not help me enough.

 

[tiny-tim]

Hi DocZaius!

If you put a weight at a distance from the axis of a non-spinning wheel, it tilts down. If you do the same at a distance of a spinning wheel, it rotates at a constant angular velocity.
…
I just can't understand why the same torque with the same dL/dt results in completely different looking phenomena.

Your terminology of "tilting down" is confusing you.

The wheel doesn't "tilt down", it rotates horizontally.

If the wheel is spinning, then its angular momentum vector is along its axis … generally at an angle below the horizontal … to make that vector rotate about a vertical axis, we need a perpendicular horizontal torque (similar to centripetal acceleration) … that torque is the horizontal (not "down" ) torque from the weight, if the weight is the correct size.

(If the weight isn't the correct size, then the gyroscope will tilt more or less to compensate … the amount of tilt will depend on the ratio of the spin rate and the weight … if the spin rate is zero, no amount of tilt will compensate.)

 

[Cleonis]

[...] I am having real trouble with precession.

I just can't understand why the same torque with the same dL/dt results in completely different looking phenomena.

Yeah. it's counterintuitive.
You are not the only one; the question you ask is a recurrent one.

Let me refer you to an earlier discussion (by me) here on physicsforums:
A post from november 2010 about https://www.physicsforums.com/showpost.php?p=2992527&postcount=3". Illustrated with images. Just a qualitative discussion.

More detailed discussion (including math) is in the http://www.cleonis.nl/physics/phys256/gyroscope_physics.php" article on my website.