Why Does a Spinning Object Wobble but Not Tumble?

[J Hawthorne]

When you spin a bowl, for instance, and it wobbles as it spins in circles. I've always been captivated at this motion, and have never been able to find the name for it. Sounds silly, but I think there's an un-tapped (as far as I know) force at work there. Thoughts? BTW, I'm definitely not in HS, just an old guy who didn't finish college. Thanks.

 

[anorlunda]

Welcome to PF.

If I understand what you mean by wobble, it is called precession.
https://en.wikipedia.org/wiki/Precession

 

[J Hawthorne]

Welcome to PF.

If I understand what you mean by wobble, it is called precession.
https://en.wikipedia.org/wiki/Precession

By "wobble" I'm talking about the undulating movement of the outside edge of the bowl as it spins...not sure if that's the same as precession or not, maybe it is. The outside edge of the bowl does a lot more "traveling" than the bowl would if it were simply spinning, like a record on a record player, correct? (Because it is also going up and down as it spins.) Do you know if this motion is used in any mechanical system or application? Also, did you write "Power is to energy as Speed is to Distance" for a reason related to this? I'm intrigued, but I'm afraid a lot of this is over my head. Hope I'm not being a pest.

 

[bigfooted]

Here's a video with some clarifying animations (applied to precession of the Earth's axis), there might be better ones. Just check if this (precession) is what you are asking about.

 

[gleem]

A rotating body can spin on its axis of symmetry while the axis of symmetry rotates ( precesses) about an axis fixed to the lab. While this is happening the spin axis can rhythmically change its angle with respect to the axis it is rotating about and this motion is called nutation.

 

[Herman Trivilino]

Sounds silly, but I think there's an un-tapped (as far as I know) force at work there.

The physics is well-understood. Do a google search for Feynman plate wobble.

Richard Feynman discusses his adventures with this phenomenon in one of his books. I think it's "Surely You're Joking Mr. Feynman" or it may be "What Do You Care What Other People Think?".

 

[anorlunda]

Also, did you write "Power is to energy as Speed is to Distance" for a reason related to this? I'm intrigued, but I'm afraid a lot of this is over my head. Hope I'm not being a pest.

No problem. It takes everyone time to get used to the site.

That sentence about power is my "signature" it goes on everything I post. You can set your own signature under preferences.

 

[anorlunda]

Does this video show the kind of wobble you mean?


Or maybe this kind?

 

[Dale]

When you spin a bowl, for instance, and it wobbles as it spins in circles.

There is spin, precession, and nutation. Spin is the fast rotation of the object about its axis.

If an object is spinning such that its axis is not vertical, then the spin axis will rotate around the vertical axis, always maintaining the same angle with the vertical as is traces out a circle in the horizontal plane. This is called precession.

Sometimes, as it precesses, the angle between the spin axis and the vertical axis will not remain constant but will oscillate up and down a bit. This is called nutation.

 

[J Hawthorne]

Does this video show the kind of wobble you mean?


Or maybe this kind?

Yes, the Euler's disc is what I'm talking about, although I use a Correlle cereal bowl; it's light weight seems to make it better suited for this motion. I wish I could find out if this kind of motion has ever been applied in a mechanical or electrical device. Seems to me, the fact that the outside edge of the bowl, traveling so much more (in up and down movement) than the bowl itself is spinning, would lend itself to some useful function...not sure if you understand what I mean.
I like your signature. Mine would be "Perpetual motion IS possible"...I'd probably get a lot of reaction to that...

 

[J Hawthorne]

The physics is well-understood. Do a google search for Feynman plate wobble.

Richard Feynman discusses his adventures with this phenomenon in one of his books. I think it's "Surely You're Joking Mr. Feynman" or it may be "What Do You Care What Other People Think?".

Thank-you, I will get that book at the library.

 

[anorlunda]

https://en.wikipedia.org/wiki/Euler's_Disk#In_popular_culture

The toy's sound and motion has been captured on both radio and film, for example, in the movies, Snow Flake[disambiguation needed] (flashing toy) and Pearl Harbor (sound for the torpedoes, also played during the Academy Awards show).[citation needed] Euler's Disk was featured on the TV show The Big Bang Theory, season 10, episode 16, which aired February 16, 2017 (MSN.com Euler's Disk vs. Raj). It is also quite popular on social media as YouTube videos of Euler's Disks spinning are often widely shared.

A signature about perpetual motion would not be welcome. Perpetual motion is a forbidden topic here at PF.

 

[Vanadium 50]

but I think there's an un-tapped (as far as I know) force at work there.

Perpetual motion IS possible

There's not and it isn't.

This was all worked out in the early 1800's by among other Louis Poinsot, giving us the wonderful yet incomprehensible expression: "The polhode rolls without slipping on the herpolhode lying in the invariable plane" The mathematical details are rather sophisticated, I'm afraid, but this all comes ultimately from Newton's Laws.

 

[J Hawthorne]

https://en.wikipedia.org/wiki/Euler's_Disk#In_popular_culture
A signature about perpetual motion would not be welcome. Perpetual motion is a forbidden topic here at PF.

"Why is that?", asked the naive newby.

 

[Dale]

"Why is that?", asked the naive newby.

Because all posts on PF must be consistent with the professional scientific literature.

 

[J Hawthorne]

OK, thanks

 

[Mark44]

"The polhode rolls without slipping on the herpolhode lying in the invariable plane"

Hoo boy! That's some arcane stuff!

I've never heard that word "plane" before.

 

[sysprog]

Thank-you, I will get that book at the library.

Feynman's explanation includes a Lagrangian transform analysis, and a deeper formulation based on the activity of the particles. It's in Surely You Are Joking, Mr. Feynman! You can find it online, but I'm not sure about the copyright status or permission, so per PF policy, I won't post a link for it here. That book is generally more narrative and anecdotal than his lectures are. The lectures have been made available here: http://www.feynmanlectures.caltech.edu

 

[RobertArvanitis]

When you spin a bowl, for instance, and it wobbles as it spins in circles. I've always been captivated at this motion, and have never been able to find the name for it. Sounds silly, but I think there's an un-tapped (as far as I know) force at work there. Thoughts? BTW, I'm definitely not in HS, just an old guy who didn't finish college. Thanks.

Perhaps you are thinking of nutation? https://en.wikipedia.org/wiki/Nutation

 

[Sooty]

By "wobble" I'm talking about the undulating movement of the outside edge of the bowl as it spins...not sure if that's the same as precession or not, maybe it is. The outside edge of the bowl does a lot more "traveling" than the bowl would if it were simply spinning, like a record on a record player, correct? (Because it is also going up and down as it spins.) Do you know if this motion is used in any mechanical system or application? Also, did you write "Power is to energy as Speed is to Distance" for a reason related to this? I'm intrigued, but I'm afraid a lot of this is over my head. Hope I'm not being a pest.

Yes, It wobbles... and it spins kinda slowly around a vertical axis while fairly ripping around in a wobbly sort of way!
"Any mechanical system or application"?
Hmmm... in a sortof way, yes!
Consider zero wobble vertically/axially, and make everything 2 dimensional - 'flat' - with the wobble being purely -radial-... look up 'Harmonic Drive' or 'strain wave gears'... a type of 'gearbox' that is sometimes used for extreme speed reduction - like for example, between a gas turbine (kinda ripping around!) and a helicopter rotor... just a few revs/second. :D
https://en.wikipedia.org/wiki/Harmonic_drive for example...

 

[scormus]

When you spin a bowl, for instance, and it wobbles as it spins in circles. I've always been captivated at this motion, and have never been able to find the name for it. Sounds silly, but I think there's an un-tapped (as far as I know) force at work there. Thoughts? BTW, I'm definitely not in HS, just an old guy who didn't finish college. Thanks.

I see the motion types - precession and nutation have already been discussed.
What is interesting is that the plate maintains a fairly organised spin, with periodic wobbles. It doesn't tumble aboout erratically. The same behaviour can be seen with a discus spinning abouts its short central axis, or a football (US) spinning about its long central axis.. Also an ellipsoid, eg a potato, spinning about either its longest or shortest axis. But if the potato is spinning about its intermediate axis then its wobbles are non periodic and will grow resulting in a tumbling motion. Why?