iTopSpin
Current Posts => Collecting, Modding, Turning and Spin Science => Topic started by: ta0 on November 28, 2022, 11:16:23 PM
-
A very curious thing about balls on turntables:
https://www.youtube.com/watch?v=3oM7hX3UUEU
I had to laugh at Steve Mould's closing remarks: "I don't know, I'm sure there is a link there. It's like spinning things, you know? I don't know, maybe."
(This is about rotating things and indirectly related to tops so I'll put it here. I might move it to NSTR later)
-
HELLO AGAIN
This is about rotating things and indirectly related to tops
indeed!! ;) isn't it the same effect as when you regenerate a spinner in a plate ?
(just enough friction to make it roll but not enough to divert from the centre of forces...)
cool effect and good document
jilm
-
With those type of things I had good fun playing at different science centers!
But thanks for posting this explanation video ta0, I haven't seriously thought about it up to now, just enjoyed playing with it.
I think it can also serve to partially explain why you often see these circular patterns on weather maps around high and low pressure centers respectively. There also the Coriolis force is of importance.
-
I didn't know this behaviour and I am very surprised. I will think to it.
-
This may explain my tendency to go round in circles rather than make progress in a particular direction. ::)
Fascinating system -- especially the 7:2 resonance. Where the heck did that come from??
Made me think of another surprising behavior: Spin a disk on a smooth, flat, surface with some friction and shove it away. Astoundingly, the spin and translation come to a stop simultaneously. Reliably.
Perhaps no connection, but will reread my 2 sources on this for clues about the turntable phenomenon.
-
Here is a video (https://youtube.com/watch?v=HvpqvrF2aR0) of that type I know from various science centers.
@Iacopo: the one closest to your place I might have seen, is in the "Muse" in Trento. I have been there a few times already, but now I am not sure that they really have it there.
@Jeremy: I do not really get what you are describing. Q&D video, maybe, please??
-
Here is a video (https://youtube.com/watch?v=HvpqvrF2aR0) of that type I know from various science centers....
@Jeremy: I do not really get what you are describing. Q&D video, maybe, please??
Words cannot express how much I want -- how much I NEED -- that turntable.
Rewatched Mould's video. Doubt a close tie now between his phenomenon and the one I mentioned. For more on the latter, look online for...
Frictional Coupling between Sliding and Spinning Motion
Zénó Farkas, Guido Bartels, Tamás Unger, and Dietrich E. Wolf
Phys. Rev. Lett. 90, 248302 – Published 18 June 2003
Seems like there could be a nifty spintoy in either phenomenon.
-
At 7:44, Mould notes that on a tilted spinning turntable, the ball follows the contour, not the fall line. A top with a ball tip spinning on a tilted surface does the same.
I had to laugh at Steve Mould's closing remarks: "I don't know, I'm sure there is a link there. It's like spinning things, you know? I don't know, maybe."
Pretty much why we're all here -- to get bamboozled by spin any way we can.
-
Here is a video (https://youtube.com/watch?v=HvpqvrF2aR0) of that type I know from various science centers.
I was not familiar with this demonstration. In the past I visited many science centers around the US and in some other countries and never saw it. Perhaps it's something relatively new or it started in Germany (the main author of the paper Mould refers to seems to be German).
The circle path itself didn't surprised me too much. It does look similar to the cyclone wind effect. But that the ratio of the periods of the turntable and the circular patch is equal to 7/2 (or 5/2) regardless of the size of the ball and the radius of the circle, really flabbergasts me. This means that the effect is not explained by just kinematics (matching relatives speeds and such) but it's dynamic. The explanation should include a torque changing the momentum of the spinning ball: the ball must behave as a gyroscope.
-
... In the past I visited many science centers around the US and in some other countries and never saw it. Perhaps it's something relatively new or it started in Germany (the main author of the paper Mould refers to seems to be German)....
Not new, don't think it started in Germany. I probably saw it for the first time around 25 years ago in a Swiss science center. They had most of their stuff based on the mother of all science centers: the Exploratorium in San Francisco.
On the Exploratorium site I find this. (https://www.exploratorium.edu/exhibits/turntable) The year 1986 is mentioned there, and alright the name "Uwe Langmesser" sounds very German.
In Germany science centers only became popular during the last 20 years or so. The one closest to my place (Phaeno (https://www.phaeno.de/en/) in Wolfsburg, Volkswagen city) is strongly influenced by that Swiss science center I mentioned earlier. Next time I visit I will definitely play longer then usually with that big turntable.
-
...
Spin a disk on a smooth, flat, surface with some friction and shove it away. Astoundingly, the spin and translation come to a stop simultaneously. Reliably.
...
I thought about it a bit and ended up by not understanding why that is "astoundingly". After all it is the same phenomenon (friction) that is stopping motions (translation, spin) that are not at all different if viewed locally from the perspective of the particles where the friction is happening.
I also looked up that paper you mentioned , Jeremy. Besides going into the math and proves and all, they also give an explanation that I am happy enough with:
"...let us try to explain qualitatively what is happening. If the velocity is much higher than the angular velocity (v ≫ Rω, i.e., ε ≫ 1), then the friction torque is negligible compared to the force, see Fig. 2(a).Therefore, the velocity decreases with a higher rate than the angular velocity, and ε decreases. On the other hand,if the angular velocity is much higher than the velocity(ε ≪ 1), then the friction torque is higher than the force,and ε increases. Thus a negative feedback effectively equilibrates the sliding and spinning motion...."
-
Not new, don't think it started in Germany. I probably saw it for the first time around 25 years ago in a Swiss science center. They had most of their stuff based on the mother of all science centers: the Exploratorium in San Francisco.
I haven't actually been at the Exploratorium, although years ago I helped them with a couple of displays on polarized light.
At the Palais de la Découverte in Paris they have a giant turntable where people seat on and roll balls at each other or even try to walk. You see the curved path of the balls and experience the Coriolis force, but you don't see the closed circle paths of this video.
-
I looked for relevant papers but most you have to pay to download. I only found these freely available:
Central drift of freely moving balls on rotating disks (https://www.researchgate.net/publication/238983675_Central_drift_of_freely_moving_balls_on_rotating_disks_A_new_method_to_measure_coefficients_of_rolling_friction)
The Rolling of a Homogeneous Ball with Slipping on a Horizontal Rotating Plane (http://nd.ics.org.ru/nd190206/)
-
I thought about it a bit and ended up by not understanding why that is "astoundingly".
I guess I'm easily astounded. And as my sig might suggest, I like it that way.
Everything in the world is strange and marvelous to well-open eyes.
-- Jose Ortega y Gasset
The authors' heuristic explanation is easy enough to understand. But arguing from the extremes of v ≫ Rω and v ≪ Rω leaves a lot of middle ground and nuts and bolts uncovered. (That's where the rather complicated math comes in.)
Still astounded that this negative feedback manages to bring the rotation and translation to a halt at exactly the same instant -- regardless of initial conditions.
-
Here is a video (https://youtube.com/watch?v=HvpqvrF2aR0) of that type I know from various science centers.
The behaviour of the disk and of the ring are also surprising, and fascinating. If I find a simple way to simulate a turntable, I will make it and see these things directly from the real. Maybe suspending a glass pane horizontally to a rope and making it spin by inertia..
But I will not try to understand what it happens exactly. The gyroscopic effect is involved, but the interaction with a moving spinning surface that at the contact point changes continuously both speed and direction seems very complicated.
-
.... If I find a simple way to simulate a turntable, ...
Maybe you have an old record player sitting somewhere?
-
...
Still astounded that this negative feedback manages to bring the rotation and translation to a halt at exactly the same instant ...
The separation of that movement into spin and translation is nothing but a mathematical trick. It is easier for us to treat it and grasp it that way, but if you think about it from the perspective of some particles of the surface of the table, at any given moment in time, there is just something sliding over it with some speed in some direction. How should those particles know how we want to separate the movement? How should they know which part of the movement they should slow down faster so that only what we call "translation" or "spin" ist left over at the end?
Its just "the" movement that comes to a halt.
-
I thought about it a bit and ended up by not understanding why that is "astoundingly".
I guess I'm easily astounded. And as my sig might suggest, I like it that way.
Everything in the world is strange and marvelous to well-open eyes.
-- Jose Ortega y Gasset
I'm with Jeremy on this one. Although the qualitative explanation is plausible and makes sense, it's still outstanding that you can rotate a disk in place with zero displacement and you can also slide a disk with zero rotation, but if you simultaneously rotate and slide a disk, it will stop rotating and sliding at the same time.
For easy reference, the paper can be downloaded here: https://arxiv.org/abs/physics/0210024
-
...
I'm with Jeremy on this one. Although the qualitative explanation is plausible and makes sense, it's still outstanding that you can rotate a disk in place with zero displacement and you can also slide a disk with zero rotation, but if you simultaneously rotate and slide a disk, it will stop rotating and sliding at the same time....
Oh, I like it that we have this controversial, almost philosophical, discussion!
I still think that much of the astonishment (that I can definitely relate to, to a certain degree) comes from the bias induced by separating the movement into "sliding" and "rotating". Just because we can describe a movement this way, does not mean that the separation has physical sense beyond making it easier to handle the problem mathematically.
Let's look only at the sliding motion for a minute:
Consider the disk only sliding along on a surface/plane in a straight line starting with some arbitrary velocity ax + by in our coordinate system where we chose the directions of x and y as we thought convenient. Is it not very peculiar now, that the movement in the direction of x always stops (due to friction) at the exact same time as the movement in the direction of y ??
-
Consider the disk only sliding along on a surface/plane in a straight line starting with some arbitrary velocity ax + by in our coordinate system where we chose the directions of x and y as we thought convenient. Is it not very peculiar now, that the movement in the direction of x always stops (due to friction) at the exact same time as the movement in the direction of y ?
But in your thought experiment, the driving force for the x and y is the same: the linear momentum. On the other hand, for the sliding and rotating disc, the sliding is powered by the linear momentum (proportional to the mass and linear speed) while the rotation is powered by the angular momentum (proportional to the moment of inertia and angular speed).
By the way, I believe this phenomenon would work for any object, not just a disc. But I guess the final ratio between the linear speed and angular speed would change. According to the paper, for the disc ε0 = v/Rω is a constant approximately equal to 0.653. It would be interesting to measure it for cylinders, tubes and cones of different height but same radius.
(https://i.ibb.co/r5pWPtG/image.png)
-
...But in your thought experiment, the driving force for the x and y is the same: the linear momentum. On the other hand, for the sliding and rotating disc, the sliding is powered by the linear momentum (proportional to the mass and linear speed) while the rotation is powered by the angular momentum (proportional to the moment of inertia and angular speed)....
But the "driving force" of sliding and rotation is in fact the same. When you look up how one arrives at the "angular momentum" you can see that is just a clever summation/integration of the little linear momenta of the constituting particles. There is no special physics for rotation, only Newton's laws summed up in ways so that you get similar looking laws as for linear movement. All special laws, forces and effects we know and love in tops and other rotating things, can be viewed as "emergent effects" based on Newtons laws (I think).
-
But the "driving force" of sliding and rotation is in fact the same. When you look up how one arrives at the "angular momentum" you can see that is just a clever summation/integration of the little linear momenta of the constituting particles. There is no special physics for rotation, only Newton's laws summed up in ways so that you get similar looking laws as for linear movement. All special laws, forces and effects we know and love in tops and other rotating things, can be viewed as "emergent effects" based on Newtons laws (I think).
All that's true, but the "emergent effects" can be very surprising as we well know on this forum.
-
Maybe you have an old record player sitting somewhere?
If I had it, it would have been the simplest solution, but I don't.
I tried with the glass pane. It was more difficult than I thought.
The main problem are the surfaces, the glass is too slippery and the tested discs slip far too much. The tested ball instead seems not smooth enough, (the noise while it rolls tells it), so its trajectory, especially when it rolls slowly, is not accurate.
I could observe some stationary rolling.
The stationary rolling is not stable, when the ball slows down a bit, (rolling resistance), it starts to move in circles.
The circles tend to be larger and larger, until falling down from the pane.
The center of these circles tended to shift towards the center of the glass pane, making difficult to observe the ball circling at a side of the center of the glass pane.
https://www.youtube.com/watch?v=otoy3FpHoKQ
-
oh oh
very cool set up iacopo,
even if the ball goes away , at least it creates a nice sound
I have a regular turntable 33 or 45 rpm
i wonder which speed would be most suitable for an experiment?
ciao for now
jim
-
oh oh
very cool set up iacopo,
even if the ball goes away , at least it creates a nice sound
I have a regular turntable 33 or 45 rpm
i wonder which speed would be most suitable for an experiment?
ciao for now
jim
Thank you, Jim. I am not sure about the optimal speed; in my case, it was difficult with the glass pane spinning fast, because I had to throw the ball without hitting with my hand the strings to which the pane was attached, but you don't have this problem, with the turntable. Maybe, also, the ball could slip towards the outside, if the speed is too high. On the other hand, I noticed that if the pane rotates too slowly, the ball also moves more slowly, and especially when the ball goes towards the center of the rotating plane, where the speeds are lowest, the ball at that point can stop to move completely and stand still, or it can move very slowly and a bit randomly in direction, because of the tiny irregularities of the surfaces, to which the ball is sensitive especially at the lowest speeds.
One interesting thing that I noticed is that, in the fourth sequence, the ball could make two revolutions staying at a side of the center of the glass pane, and in that situation the ratio between the rotational speed of the pane and that of the revolutions of the ball was about 7:2, as explained before in the thread. Intriguing.
-
I love this discussion so interesting and visually mesmerizing