# I Resistance and Precession Torque of a Gyroscope

#### dishwasher95

Summary
What is the approach to determining the resisting and precessing torque of a spinning gyroscope?
Hi,

Say there's a gyroscope with moments of inertia Ix, Iy and Iz spinning around a vertical z-axis (see attached illustration) with a given angular velocity ωz. Notice that the gyroscope is floating in space as in that there's no gravity acting on the gyroscope.

Now I apply a torque τx around the x-axis.

The torque will introduce angular momentum Lx around the x-axis.

What I want to know is, how can I determine the resisting torque τres,x that resists the motion of the applied torque τx?

And what is the size of the torque τy that will occur due to the applied torque τx?

I'm look for an analytical approach to relate the applied torque τx to τy and τres,x.

I feel like I've searched the whole internet for a clear explanation but failed to find one.

If anyone would be willing to go through the theory step-by-step or just show me some literature that does I'd appreciate it immensely!

Attached is also some formulas I found online but I would love a derivation.  Related Classical Physics News on Phys.org

#### A.T.

What I want to know is, how can I determine the resisting torque τres,x that resists the motion of the applied torque τx?
What to you mean by "resist the motion"? If you apply τx to the gyroscope, the gyroscope applies -τx to you.

• dishwasher95

#### dishwasher95

I'm not really sure how to describe the 'resisting torque'.

I'm not thinking about Newton's 3rd law.

I feel that whenever I try to apply a torque to my spinning gyroscope, it tries to resist the motion around the applied torque's axis. But I'm not 100% that this is what is happening. Maybe it just 'feels' as if there's a resisting torque because the gyroscope starts to rotate around the y-axis with the induced τy?

So if there is no τres,x (of course there's the equal but opposite reaction -τx that you mentioned) do you have any idea what the 'Resistance Torque' formula in the attached table is?

I still want to know how to relate τx to τy though.

Thanks!

#### A.T.

do you have any idea what the 'Resistance Torque' formula in the attached table is?
Maybe you should post a link to the source.

#### A.T.

Sure thing! Here you go:
New Analytical Approach for Finding the Gyroscope Forces and its Properties
Here's another article talking about the same formulas:
Gyroscope Mystery is Solved
Both is from the same author. Neither shows any derivation, just the formulas. The torque predictions disagree with the standard approach based on on angular momentum conservation (2nd link, Eq 6 vs Eq 7). The explanation of this disagreement based on centrifugal and Coriolis forces doesn't make much sense to me.

• dishwasher95

#### dishwasher95

I'm glad I'm not the only one who didn't understand it - thought I was missing something.

But let's forget about the articles, τres,x and the formulas from the table.

I'm interested in finding a relation between τx and τy.

I found this website (see the bottom 'Mathematical Discussion'):
Gyroscope physics

It seems that the 'Mathematical Discussion' derives τy (which in the article is just τ) from ωxs - the swivelling rate in the article).

I think this might be what I'm looking for - what do you think @A.T. ?

I have some trouble understanding how the 'tendency to pull ahead/lag behind overall swiveling' (F = -2mωsvr) is supposed to be understood. I think I need a more intuitive explanation of what is happening.

#### A.T.

I'm interested in finding a relation between τx and τy.
If you apply a torque around x, then τy = 0.

I think this might be what I'm looking for - what do you think @A.T. ?
I don't know what you are looking for. Here is how the applied torque relates to the procession rate:
https://en.wikipedia.org/wiki/Precession#Classical_(Newtonian)

I think I need a more intuitive explanation of what is happening.
For intuition it might help to look at it in terms of linear motion:

#### dishwasher95

I must apologise for any confusion. Let me try again:

The wikipedia article that you linked was very informative. I'll use its description to explain my problem.

After you've read the text, notice the red box in the bottom with the marked word 'torque'. THIS torque (as it is described in the article) is the one I'm looking for. This torque is my τy. The pitching torque.

The article states that the pitching torque arises from the Coriolis Force (marked with blue squares in the screenshot).

The only question I have now (after newly gained information) is how the Coriolis Force can be calculated. From the article that I posted before:

'Gyroscope Physics' (at this moment the website is down?)

- the Coriolis Force is derived to be:

F = -2 * m * ωs * vr

- where m is the mass of a point on the rotating disc of the gyro
- ωs is the spinning angular velocity
- vr is the radial velocity

I understand what the Coriolis Effect is, but I have trouble understanding how the Coriolis Force is derived.

So here's my (updated) question/wish:

For someone to show me an intuitive and relatively simple derivation of the Coriolis Force F = -2 * m * ωs * vr

Thanks!

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#### jbriggs444

Homework Helper
I understand what the Coriolis Effect is, but I have trouble understanding how the Coriolis Force is derived.
Let us verify that we have common ground for discussion.

The Coriolis effect does not arise from looking at rotating objects from an inertial point of view. It arises from adopting a frame of reference that is itself rotating. This is often convenient because it means that we can look at a rotating object as if it were not rotating.

Let us bring that Wikipedia article in as text rather than as an attached image...

Wiki said:
To distinguish between the two horizontal axes, rotation around the wheel hub will be called spinning, and rotation around the gimbal axis will be called pitching. Rotation around the vertical pivot axis is called rotation.
This is a nice choice of terminology.
Wiki said:
First, imagine that the entire device is rotating around the (vertical) pivot axis. Then, spinning of the wheel (around the wheelhub) is added. Imagine the gimbal axis to be locked, so that the wheel cannot pitch. The gimbal axis has sensors, that measure whether there is a torque around the gimbal axis.
In order to invoke Coriolis, we need a rotating frame of reference. So we pick one which is locked to the uniform rotation of the device around the vertical axis. In this frame of reference the device is no longer rotating. It is just sitting there and spinning in place.

But this is a rotating frame of reference. In a rotating frame of reference, angular momentum is not necessarily conserved. We can have a changing angular momentum with no external torque. Or we can have constant angular momentum with a non-zero net external torque. The claim is that the latter is the case here -- we have a wheel that is rotating in place but only so long as a net external torque is supplied.

The task is to calculate the required external torque. Coriolis allows it to be calculated. It turns out to be a pitching torque.

#### A.T.

- the Coriolis Force is derived to be:

F = -2 * m * ωs * vr

- where m is the mass of a point on the rotating disc of the gyro
- ωs is the spinning angular velocity
- vr is the radial velocity
Note that ωs is the angular velocity of your reference frame (around the vertical pivot axis), not of the rotating disc. And vr is the velocity component perpendicular to the vertical pivot axis. The Coriolis force and its lever arm around the pitch axis are not constant along the disc, so to get the total Coriolis torque you would have to use integration.

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