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19 August 2019 14:10

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Asked by: Nitro
Subject: precession around the centre of mass
Question: Hi all,

Blaze and others have suggested that an overhung gyro on a frictionless base will rotate around its centre of mass

Blaze, you are fundamentally wrong in your assumption that a pivoted overhung gyro will rotate around its barycentre if mounted on the proverbial frictionless surface (air table). There is a typical example of a “scientist” carrying out a test to “prove” their hypothesis, rather than get to the truth, in a bad video to be found here:-


This was created by Emma Wilson in 1997 as part of her MEng thesis at Cambridge University under Dr Hugh Hunt. Shame on her for this poor work!

At first glance it would appear that your (and her) view is correct (a real scientist should not be fooled by a first glance). However, on further reflection it will become apparent that the reason the base appears to be rotating as you suggest it should, is because the non rotating mass (gyro frame,inner mass, shaft etc.) has not been counterbalanced and it is this lack of counter balancing that is causing the base to counter rotate. If you look at the relative mass of the heavy gyro compared to the relatively light base it becomes clear that, if your view and hers were true, the base should be moving much more than the gyro not the other way round. The rotating gyro component will not cause a counter rotating force on the pivot point.

A simple test without an air table can confirm that rotation occurs around the pivot point and not around around the centre of mass.

First extend the gyro’s axle beyond its pivot point and carefully balance the gyro’s NON ROTATING mass. Suspend your over hung gyro from a thread at its pivot point. Allow it to stabilise and use a pointer to show its stable suspended pivot point. Use another thread to suspend what will be the free end of the gyro so that the axle is horizontal. With the gyro spun up carefully ensure that its suspended pivot point stabilizes and is aligned with the pointer. Burn through the thread suspending the free end so that you don’t influence the motion when the gyro is released to precess. Carefully observe what happens and rearrange your understanding accordingly.

It is because of this type of anomaly and that a gyro enables the separation of angular and linear momentum that makes inertial drive possible. Whether it will turn out to be practicable remains to be seen.

I don’t want to take time at the moment to make and put up a video of this simple demonstration device but, if necessary, I will once I have completed my resurrection of the “fast repeater” which, not surprisingly, I feel should receive greater priority.

Kind regards

PS My observations on this have, as usual, been made by the late great Eric Laithwaite and are shown on this very site here:-


It can clearly be seen that far from rotating about its centre of mass the gyro precesses OUTWARD from its spin centre.
Date: 12 May 2013
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Answers (Ordered by Date)

Answer: Nitro - 13/05/2013 13:14:48
 Hi all again,

Sorry! That last sentance should, of course, have read ....OUTWARD from its pivot center.

Please see the video.

Kind regards

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Answer: Momentus - 13/05/2013 14:27:23
 Hi Nitro,
The Emma Wilson proof of Lathwaite’s ice tower experiment is indeed a damning commentary on Big Sci. I did write to Hunt, (rhymes well), a long time ago, but got no reply.

I analysed the video and determined that the centre of rotation is 11 units from the centre of the gyro, and 1 unit from the centre of the tower. Those of an advanced technical nature can work out the respective masses and decide for themselves.

You have described the basic experiment. It shows not only the absence of radial force, but can also, by adjusting the balance weight, show a “negative” radial force.
By observing a video the instantaneous start of the tangential motion can be seen, showing the absence of force in that direction of motion.

It was for a long, long time the place I returned to when the “experiment that would work this time” failed.

Study of this simple apparatus led me – eventually to Dark Motion, by testing each new concept as an explanation for this behaviour.

My outline paper on dark motion still sits upon the desk of an eminent professor, Head of the physics department of a major UK University. As yet he has found no fault. As yet he will give no endorsement.

Sooner or later I will get pissed off, but this currently remains my best bet.


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Answer: Nitro - 13/05/2013 15:44:16
 Hi Momentus,

Thanks for that. Welcome to the Dark Side.

Kind regards

PS That you have a good command of Rhyming slang and add "off" to pissed means you are British. Hopefully with all the chaos Ukip have stirred up we may again get to be, with all the people that are here, Great again one day.

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Answer: Blaze - 27/06/2013 01:14:58
 Hi Nitro. What you are describing with the gyro on a string is not the same scenario as I was referring to when a gyro and pivot rotate around their barycenter. In the case of a gyro on a string, the gyro is pulled outward due to centrifugal acceleration. There is no pivot weight to speak of, so no barycenter (before you jump all over this sentence, read the next one and the next paragraph). As soon as there is any circling motion, the centrifugal acceleration takes over and the gyro is pulled outwards.

By the way, how closely have you looked at that video of the professor with the gyro on the string? If you watch closely you will see that the gyro axle rarely points towards the center of the cone but rather it points ahead or behind the center of the cone. That means that the gyro is, at times, rotating about its center of mass, sometimes clockwise and sometimes counterclockwise (when viewed from above, actually it oscillates back and forth as it precesses). Do you understand why? It makes perfect sense when you do.

There is a lot going on in the video that seems to be missed or overlooked by most on this forum.


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