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19 July 2019 20:35

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Asked by: Blaze
Subject: What happens when you twirl a gyro very quickly on the end of a string?
Question: Everyone knows what happens when you hold your arm over your head and twirl a rock on a string. No matter how fast you twirl it, it will never quite get to horizontal although it will get close. Now replace the rock at the end of the string with a well spun gyro and twirl it really fast. Assume that when looking down from above that the string and gyro are moving in a clockwise pattern. Assume that when looking from the outside of the system that the gryo flywheel is turning clockwise.

Now, what motion does the gyro make? Does the gyro axle just stay in the horizontal plain (or very near to the horizontal plain)? Or does it do something else?

The end of the gyro axle not attached to the string will actually revolve in a small circle in the vertical plain. The direction of that circle will be clockwise when viewed from outside of the system. Furthermore, the gyro axle attached to the string will also revolve in a clockwise circle in the vertical plain when viewed from outside the system. In other words, the two axle ends of the gyro make a cone shaped patterns with the tips of the cones at the midpoint of the axle which is the center of mass of the gyro flywheel. To do that the string also has to make a cone in the horizontal plain (cone is laying sideways, not standing up) with the tip of the cone at your hand.

Your thoughts?

Date: 13 April 2014
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Answers (Ordered by Date)

Answer: Blaze - 13/04/2014 14:17:31
 Nitro's response:

Dear Blaze,

Interesting observation.

I think that the original rotation of the gyro axis having its axial angle changed by being rotated on the end of the string would tilt the axial angle upwards. Having raised the axial angle upwards, the centrifugal force now trying to level the axial angle will cause a precessional change of the axial angle in a rearward direction and the centrifugal force trying to straighten the axial angle is precessed .... and so on. Thus the continuously changing axial angle causes a continuously changing direction of the centrifugal force which causes a continuously changing axial angle etc. etc. to give you the twin coning effect. I would expect the cone to be angled slightly upwards. Nitro first law is everywhere.

Is the twin cone centred on the centre of mass? Quite why it should be a twin cone (from the centre of mass?) someone else can have a go at.


PS Blaze et al, I am sorry to answer your question with a new string but, since my computer got replaced, the answer form keeps rejecting my submissions because, it says, it has a wrong e-mail or security number yet, as you can see, I can start a new string because no number is required. Any ideas why it rejects my replies and dont just say its because my replies are crap you utter shower! And sit up straight when Im writing to you!
Date: 13 April 2014

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Answer: Blaze - 13/04/2014 14:31:36
 Very good Nitro. You got the idea but I would describe it as so:

The upwards tilting (outer) axle end is caused by forced high speed precession (twirling the string quickly well above its "natural" precession rate). This means it starts to move upwards, which causes it to precess backwards and at the same time as it is moving upwards and backwards the centrifugal force takes causes it to move downwards which precesses it forwards, etc, etc, etc. Which I believe is saying the same thing in a slightly different way.

By the way, I find your answers are usually pretty good.


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Answer: Glenn Hawkins - 14/04/2014 18:09:02
 Hello Blaze & Nitro,
This happens when you force a gyro faster than its normal precession speed around a center point, whether by finger to pedestal, or whorled string, it rises for the same reason.
Cheers, Glenn

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Answer: Blaze - 15/04/2014 04:02:33
 Hi Glenn. I think maybe you missed some of the question. The gyro will rise initially but will not stay there. Due to centrifugal force, the whole gyro will rotate about its center of mass or very near its center of mass with the axle ends describing the bases of cones and the tips of the cones pointing to the center of the gyro axle.


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Answer: Glenn Hawkins - 30/04/2014 20:44:09
 Hello Blase,
You are right, I read your message to fast. I should know better when it comes to gyroscopes. I just finished a long and difficult project and I am still dragging and will be for a few days.

I lost the reply I had done. Curses! Precession is a series. Each movement causes a right angle movement, or rather another precession. Until the last precession is blocked by the force that cause precession in the first place; i.e. if gravity forces a gyro down, precession occurs at a right horizontal angle, then that precession cause an upward right angle precession attempt, BUT upwards it meets the downward force of gravity and therefore cannot move upwards. I know you know this, but for others who might not, I explain.

When your string gyro attempts to wist upward as precession would have it; the upward twist attempts to twist backwards from the string twirling direction, which would cause it to twist downward thus continuing the series of right angle twisting. It is unstable and cannot have an equable stillness while in motion and so it's action must wobble. Consider that an out of alinement auto tire must wobble, though not for the same gyroscopic reasons.

I think the other explanation was better, but is this one clear?

Regards Glenn,

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Answer: Blaze - 01/05/2014 00:48:26
 Hi Glenn. Yes I get the gist of your explanation which I believe agrees with Nitro's and mine.


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Answer: Blaze - 06/05/2014 02:24:12
 Now for the surprise. When I came up with this idea, I was going to try it and video it but then I remembered that, in a fashion, it has already been done.

In the Christmas lecture video number nine, "15-16lb gyroscope hanging from a long piece of string" you can actually see what I have described happening. You have to look closely because the gyro is not twirling about on the string quickly, but rather quite slowly, so the end of the gyro axle not attached to the string revolves in a small circle in the vertical plain quite slowly as well. It takes several complete precessions of the gyro for the end of the gyro axle not attached to the string to revolve once in a small circle in the vertical plain.


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