The Bizarre Behavior of Rotating Bodies, Explained

by **LongtimeAirman** Tue Oct 01, 2019 4:26 pm

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The Bizarre Behavior of Rotating Bodies, Explained

Spinning objects have strange instabilities known as The Dzhanibekov Effect or Tennis Racket Theorem - this video offers an intuitive explanation.

I was perusing our good buddies at cutting through the fog and was delighted to find a post by haggisnneeps, the link to this youTube video. I hope he doesn’t mind me thanking him by reposting him here.

https://cuttingthroughthefog.com/2018/08/28/space-fakery-the-final-frontier/comment-page-8/#comment-24477

haggisnneepssaid:
September 27, 2019 at 11:28 am
I just came across this on t’internet and wondered if there was/is a better explanation for this effect as the explanation given doesn’t seem to be very rigorous….The Dzhanibekov Effect

For me, early on in the explanation we are told that spheres only have 1 moment of inertia and by the end of it we are told that planets all spin along their major axis or moment of inertia

Surely, then, the experiment earlier would have been to add some weights/mountains to the outside of the spherical ball and spin that the same way?

Also the CGI graphics state that there needs to be an initial cause of acceleration on the minor axis/moment of inertia but if you watch the spinning wing-nut (cgi) or spinning T-Bar, there is nothing /no force happening to initiate this extra spin.

There also must be some correlation with the number of rotations per flip

Enjoy….answers on a postcard to NASA…..

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by **Jared Magneson** Wed Oct 09, 2019 8:43 pm

I made a simple video demonstrating this as well.

https://vimeo.com/364227191

It turns out if you have any deviation at all, it will cause the nutation and the flip-spin. The only way to avoid it is to have a perfect single-axis outward spin, which using human hands and human-made machine threading is highly unlikely to occur. If you did it a thousand times you might get it right a few times, obviously, but that margin of error is pretty high.

by **LongtimeAirman** Wed Oct 09, 2019 10:20 pm

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It turns out if you have any deviation at all, it will cause the nutation and the flip-spin

That’s an excellent animation Jared. I can easily agree with your description and assessment – any imbalance in the thrust exerted on the opposite sides of the threads will result in an intermediate-axis rotation. Please accept one criticism, the handle seems to be constantly drifting toward the viewer, forward as well as sideways, the handle’s threads seem to travel sideways past the thread wall axis before the threads clear the surface.

But what about the awful global implications. At about 10 minutes in we learn that when the handle is encased within modeling clay the sphere will also perform a flip spin. All proton matter – i.e. astronomical bodies that recycle photons are slowly redistributing mass on/in that body. Do all unbalanced spherical bodies periodically flip? It could help explain Venus.
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by **Jared Magneson** Thu Oct 10, 2019 1:11 am

Well if all unbalanced spheroids did, the Earth and every other planet would as well. No, they spin the directions they do because of their charge/anticharge ratios. Venus is still slowing down to match hers, as far as I know. Her position is the newest it seems, for whatever reason.

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The Bizarre Behavior of Rotating Bodies, Explained

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