The Turntable Paradox

[@SteveMould]

I can't believe I didn't make a "how the turntables" joke. That's why I love the comments section! Here's a paper with the calculations: m2.askthephysicist.com/Weltner.pdf Note that equations 18 and 19 should have R² terms. That threw me off for longer than I care to admit!

What happens with shapes of constant width? My guess is it depends on it's moment of inertia, but still would be fun to see.

[@isaacm1929]

Spinning balls. Balls on the table. You could've done so much jokes... Have a great day sir! Also, Baller, Balling.

[@brianm6337]

Does this work with non spherical solids, too? How about a water filled ball?

[@calholli]

It's hilarious to say that the car is pointing "tangentially"-- When in fact, no matter what direction you point the car on a round table, it is pointing OUTWARD; lol.

[@fookingsog]

Our family used to have a membership to a children's museum where they had an exhibit similar to this. The spinning disc platter was probably 3-4 feet in diameter stainless steel. They had a variety of different diameter and thicknesses of disks that you could experiment with and balance on the big spinning disc platter. Was probably my favorite exhibit as it required intuitive interaction!!! Super Fun!!! 😁👍🏻

[@weeeeems]

I'm gonna need a 2-dimensional, transparent, liquid filled representation of this.

[@lyricsassam]

😂😂😂

[@furzekoenigin]

It's the only way for me to understand these things.

[@swedishpsychopath8795]

Could we settle for a venn-diagram instead? So Kamala will understand too?

[@appalachianforge7537]

Sucks for you

[@philipfahy9658]

We used to use a tank on a rotating table like this with food coloring to simulate solid body rotation in undergrad. So 3d, but transparent and liquid filled, we're almost there!

'discs behave wiredly on turntables'... that sums up my entire experience of the 90's quite nicely

[@scottmatznick3140]

Weirdly

[@SirNobleIZH]

Wiredly

[@ferrumignis]

You can get Bluetooth turntables now, so the discs behave cordlessly.

[@fluffigverbimmelt]

"90s" "turntables" "behave wired-ly" Hm, am I overthinking this?

[@Ohmanwhyyourfeelingshurt]

You spun them the wrong way

[@WinEntity]

This is the first time in a long time that I've genuinely felt fascinated by the application of mathematics as hard-and-fast rules for how our world works. Thank you for this fascinating journey.

[@tshddx]

Would be cool to mount the top-down camera to the turntable so it rotates with it. When you mentioned the non-inertial reference frame stuff I was hoping to see the ball’s path from that reference frame.

[@dustinandrews89019]

Yes, I very much am interested in that. My intuition is that it's elliptical orbits that resemble orbits due to gravity. Though, maybe it's more like anti-gravity since the force on objects is outward unless they can roll.

[@mrjbexample]

-Look up hypocycloids, they are star-like shapes that form if you trace a point on a small circle rotating within a larger circle. I think the ball's path from the table's reference frame would be look like one, with k=3.5 or 7 points to the star.- EDIT: this is wrong, it will just be a spiral that moves inwards then outwards and joins up with itself

[@jetison333]

its possible to just stabilize the video on the turn table, if someone wanted to put in the effort.

[@PhysHow42]

@@dustinandrews89019 he mentions in the video that it cuts a perfect circle in vacuum without slippage, and a spiral with slippage.

[@MrDaraghkinch]

@@mrjbexample spirograph?

[@jaredarnell]

Love the video! Wanted to say, I did my master's thesis on how students conceptualize the Coriolis force, and I'd recommend avoiding terms like "fictitious" when describing it. It gives students the impression that it's 'made up' or 'doesn't exist', which conflicts with their bodily perceptions which have experienced the force first-hand. Also, it makes it sound like it shouldn't be trusted (let alone, used), rather than emphasizing how helpful (and necessary) the Coriolis force is when viewing things from a non-inertial frame. Personally, I try to call it an "apparent" force, because it 'appears' when you change your perspective to the non-inertial frame. It's all about clarifying the contexts in which the Coriolis force is productive.

[@SteveMould]

That's a really good point, thanks! How about virtual force? I quite like "apparent" though.

[@insidejazzguitar8112]

@@SteveMould you could call it the Coriolis effect

[@PeskyTheWabbit]

@@SteveMould One of my hobbies is long range rifle shooting, so as you could imagine, the Coriolis Force / Effect becomes a factor in my accuracy. When you said "fictitious force" in the video, I definitely raised an eyebrow before I reminded myself that centrifugal force is similar in not being a real "force", because I have definitely witnessed the Coriolis Effect. I feel like "fictitious" can cause a knee jerk reaction. I think "apparent" would fit perfectly, or simply referring to it as the Coriolis Effect, as that does not imply any "force" is being added to the equation.

[@jannegrey]

@@SteveMould Especially since Flat Earthers will use it as "proof" that Coriolis effect isn't "real". Apparent force is much better. Same for Centrifugal Force.

[@LeoStaley]

@@SteveMould "emergent" is often used to describe effects which are not fundamental, but arise as a consequence of other more fundamental effects.

[@freddym99]

Well well well, how the turntables....

[@past-tense-of-draw]

Underrated comment

[@DrAimanAKhan]

There’s always that one guy 😂

[@tamsinashton2293]

How indeed 🤔

[@soup_slayerShrek]

"oh how the turns have tabled"-my brother, 2024

[@ChandrasegaranNarasimhan]

Will will will, how the will arches.

[@MyriadCelestia]

as soon as you brought up a hollow ball and I saw the numbers 5 and 7 pop up, I thought of Moment of Inertia. I am pretty proud of my tiny noggin for thinking of that

[@davidetosches6014]

same. I also thought at Cv and Cp of ideal gases but then I excluded then because of the context (though, it's still spinning stuffs and inertia ;) )

[@MyriadCelestia]

@RestWithin i am probably younger than you think haha

[@squigglycups5666]

​@@MyriadCelestia atleast 80

[@ezfzx]

It is precisely related to the inertial moment, confirmed if you examine the paper. We try to express the equation as: I = k m r², with k = 1 for hoops, of course. We have a lab the student do that rolls different types of balls down a rail of variable separation and they have to predict the speed in terms of the "k" value. It's very enlightening. As a lab, this would be equally fascinating.

[@MyriadCelestia]

@Josias Lourenço ummm

[@carljohanr]

I saw this at the Experimentarium, a Science Museum in Denmark. One interesting variation is a large hollow ring (a thick bracelet or similar). If you put it on the turntable vertically and let it get up to speed, and then place a ball inside the ring, the ball inside will stabilize the motion of the ring, and behave quite similarly to a single ball.

[@AlDunbar]

Most of us would, I think, be surprised to see a ball on a turning turntable basically staying in one spot. Instead of explaining with formulas only, you described how it happens very simply. Great work!

[@luizucchetto2528]

Your videos are simply awesome! I am a rerired Physics Teacher and could have used your videos to engage and challenge my students while I was teaching. I never miss your videos and thank you for keeping my love of Physics alive and I hope inspiring a whole new generation of young students to take up the challenge of physics and science in general.

[@aze4308]

your videos

[@JoshHenderson16]

The hollow ball discrepancy blew my mind. And the seeing the mathematical proof was so satisfying. I love when maths describes real world phenomenon so comprehensively.

[@Gisburne2000]

Watching this video almost 40 years after I dropped out of taking Physics and Maths for A-level, I'm glad I did. The overlying description of what's happening is utterly fascinating, but the calculus, the physics, the number-crunching... it was never going to be for me. I absolutely LOVE the passion you and others have for such things, because I have my own 'things' which give me joy. Glad to be a viewer, amazed to catch of a glimpse of something described in a way I can understand it, knowing that you can do the maths and the physics and I don't have to!

[@frightenedsoul]

There was a time around 6th-8th grade learning high school level algebra with a very good math teacher that I really enjoyed it mostly because the teacher made me understand it and therefore I was really good at it and had really good grades as a result. Fast forward several more years and terrible math teachers later and I was never able to successfully advance past that and had terrible math grades by the time I got into college.

[@occupyallthethings]

Lol I'm the opposite.. didn't drop out and got a mechanical engineering degree and lots of regret

[@dark6.63E-34]

@@occupyallthethings can you elaborate? I am a first year in mechanical engineering and maybe i could get some insight

[@shet3930]

this is a typo and you meant to say 4 right?

[@thitam5003]

ok

[@TimeBucks]

Your videos are simply awesome

[@evaakter6425]

Very helpful

[@supriyanto8401]

Thumbs up

[@nazkhan3506]

👍

[@ethanmanman8867]

Hello verified user

[@misssusmita2502]

Very good

[@batlin]

Would have loved to see a view locked to the turntable's rotation (i.e. a camera from above rotating at the same speed, or each frame rotated to keep the turntable apparently in a fixed position). Bet the ball movement would look pretty interesting.

[@trif55]

This would be great! Also this video's a decent explanation of orbital mechanics, where poking it inward to where it's moving faster etc

[@davidwuhrer6704]

Seems simple enough to deduce what it would look like. For the ball staying in the spot, it just describes a circular orbit for the table. For the ball going in a circle, it would be an elliptical orbit precessing one seventh with each turn, so that after seven orbits it ends up where it started again. Or five if the ball is hollow.

[@batlin]

@@davidwuhrer6704 it's not that I can't figure out what it would look like, but it would be enjoyable to see.

[@davidwuhrer6704]

@@batlin That it would be.

[@blaise3004]

I can't be the only one that initially thought it was pi rotations rather than 7/2 when you counted them

[@sudoscoobs1373]

Same! I was sitting here like, "do I smell pie?"

[@isaacm1929]

I was also thinking pi rotations!

[@dav1dsm1th]

@@sudoscoobs1373 Great. Now I'm hungry.

[@NourSelim0]

same, it's a very likely place for pi to come jumping at you 😁

[@DrakiniteOfficial]

Sad to say I didn't think of that. I was just confused at how the heck 7/2 seemed to appear.

[@trustnoone81]

I started chuckling to myself the second I saw the equation for the moment of inertia of a ball. This was so cool! It's nice to see some interesting physics can still be done with closed form equations.

[@lukedowneslukedownes5900]

I’m curious. What is your thoughts on open and closed formulas relative to physics?

[@dickJohnsonpeter]

@@lukedowneslukedownes5900 Equations are closed, formulas are open.

[@ratulxy]

@@lukedowneslukedownes5900 by closed form equations they mean that the equation can be solved analytically which generally leads a nice solution. Otherwise you have to solve it numerically.

[@pineapplepenumbra]

@@ratulxy It helps that your post was good, but I thumbed you up before I read it, just because your name deserves it.

[@ratulxy]

@@pineapplepenumbra haha, thanks!

[@ignispurgatorius5297]

I like that you showed some of the math here as well. I think too many youtube science channels forget that besides just describing observations we also already have a lot of very good models that can predict the observations very well.

[@lasagnahog7695]

Classic Mould. Breaking an interesting phenomenon down to easily understandable parts. It makes me feel smarter than other channels because it's like he is just making me realize what I already know opposed to teaching a whole new concept.

[@paulshuttleworth6261]

Also very willing to admit he doesn’t fully understand it either. Humanises him. Great guy.

[@rgw5991]

OK

[@chrismofer]

This helps me visualize how a Lagrangian orbit can be somewhat stable despite all the forces being apparently unblanced.

[@pineapplepenumbra]

Good point.

[@ThreesixnineGF]

Yeah that's kind of how JW telescope keeps orbiting around it's lagrangian center. Although its path is an 8 figure rather than a perfect circle. Which make me think that this 8 figure (lemniscate) might be a 3D substitute for the 2D circle. But idk

[@JamesHawkeYouTube]

The sky is not "outer space". Sorry to break the sci-fi illusion. "Gravity" is a pseudo-scientific belief about balls of matter in the sky.

[@brunnomenxa]

@@pineapplepenumbra, Good Lagrangian point.

[@Astromath]

That's exactly what came to my mind immediately when he showed the circular orbit of the ball that doesn't go around the center!

[@armanmohsenikabir7293]

Amazing video. I have a doctorate degree in Physics and I am always amazed with your videos. Kudos and thanks for the link to the paper.

[@sphygo]

I swear you always come up with the most mundane topics that slip right by the rest of us, and yet are so very fascinating when you take a closer look at them. Another amazing video.

[@oskioskioski]

A ball rolling from a flat surface, onto a turntable, back onto a flat surface, is also interesting. It swerves on the table, but exits perfectly in line with the direction it entered from.

[@erikeriks]

Modern scientists: it exits perfectly in line with the direction it entered from. The line in question: 5:32

[@Call_Upon_YAH]

Jesus Christ is the propitiation for the whole world's sins. They that believeth and are baptized (with the Holy Spirit) shall be saved; but they that believeth not shall be damned. Those led by the Holy Spirit do not abide in wickedness. 👍🏾 *God is ONE manifesting himself as THREE;* the Father, the Son, and the Holy Spirit! Bless him! *For these three are one.* As I am led by the Holy Spirit, nothing I state is a lie, but the truth of God. Anyone who tells you differently is misinformed or a liar. They do not know God, nor led by him. Anyone who *claims* to be a Christian and is against what I am doing, and where I am doing it; the Holy Spirit does not dwell within them, they lack understanding. They know not God, read his word, and their religion is in vain. Do not hear them, they will mislead you, the lost cannot guide the lost.

[@Call_Upon_YAH]

When you trust in God and cast your cares (worries, anxiety, depression, suicidal thoughts) upon him, they will be NO MORE! Know that there is power in the name Jesus Christ! His name casts out demons and heals! The world is wicked, evil, and of the devil. I too, was a wicked sinner of the world before I opened my heart to God. I am living proof of God's work and fruitfulness! He is an active God who hears the prayers of his! God's children are set apart (holy) and righteous. The devil is a liar that comes to steal, to kill, and to destroy; that includes your relationship with God. Open your heart to God, repent of your sins (he will forgive you), and let him direct your path. Draw nigh to God, and he will draw nigh to you. Cleanse your hands and purify your heart, lest you walk with the devil and follow him to hell.

[@GABSE007]

The Linik you are talking about in 8:14 is the change of angular momentum-> you can use the right hand rule to obtain the direction of deflection Example aircraft with one jet engine : Your right hand "wrap"-fingers are pointing in the direction of the engine's rotation-> your thumb is now pointing out of the engine Lets assume the jet is pulling nose up Imagine the line that your thumb draws now use right hand rule again-> your thumb is pointing in the direction of the change (up) and your "wrap" fingers show you in which direction the moment is introduced So in this case the airplane would yaw to the left (if the pilot is sleeping)

[@adrianvankan7619]

Amazing video, as always! I've never commented here before, but I'm a long-time subscriber and wanted to say that as a researcher in physics, your curiosity is simply an inspiration to me! Also, I love your plug for THE LÄND (Baden-Württemberg), which is where I did my undergrad. It's a great place for Science indeed!

[@danielsieker9927]

Let me guess, Heidelberg?

[@adrianvankan7619]

@@danielsieker9927 That's right!

[@Bob_Adkins]

I imagine the formula for the motion of a slightly elliptical ball would be terrifying.

[@lucasbakeforero426]

I am getting nightmares already.

[@StigFerrari]

It can’t hurt you

[@santoslkwjs4718]

yesss kzbin.info/www/bejne/mH7bfqiYi9Zma5I

[@shadowcween7890]

The truth is that any real world ball is already slightly elliptical, since perfect shapes are nearly impossible to create

[@daty1453]

​@@shadowcween7890 why are u saying nearly

[@RohitDee49]

Correct me if I'm wrong, but shouldn't the ball be pushed outward because of centrifugal force contrary to what Steve says at 2:43?

[@boing7679]

But the ball is not spinning arund the turntable, so there is no centrifugal force being excerted on it.

[@twojuiceman]

When I was a kid, the Fort Worth Science Museum had a 6-ft radius stainless steel turntable set flush in a table so there were no pinch points around the edge. They had all manner of round objects for kids to play with. I remember my parents saying it was time to move on, but I was mesmerized playing with wheels and balls on the spinning table. Good times

[@DaHoodedBandit]

I love how you can see the line bent with the rolling shutter effect when he pauses the video. So cool

[@levetbyck]

this part 3:08!

[@bobwilson00]

from 3:08 to 3:29 you can actually learn something about cameras as well, where each frame is taken from the top to the bottom, so the bottom of the image is slightly later than the top. This makes it look like the perfectly straight line on the turn table is curved, since it has had more time to rotate the further down it gets. You can also see that when it rotates close to being horizontal this curve goes away since the horizontal lines are taken a lot closer together chronologically.

[@zacharyhenning6854]

Would love to see a camera view mounted to the turn table. I assume the the ball would appear to trace a pattern like a spirograph.

[@better_dead_than_red]

I got a nausea from this idea

[@levetbyck]

maybe have a sensor inside the ball

[@RoyceRemix]

As soon as I saw your comment I knew I had to make it happen - Here's a handful of shots from the video that I tracked, and one shot that I traced the path of the ball, I put my thoughts about it in the description under the video (unlisted, but still sharable) - kzbin.info/www/bejne/Y5q0e6CGpJanitU

[@adamplace1414]

I don't think I've seen a Steve Mould video I didn't like, but this might be my favorite. I was slack-jawed in surprise for a lot of it. The way the balls moved really was unexpected and quite pleasing as well.

[@AstarOfDavid]

So what happens if you tried using a ball with an offset center? Like a bolwing ball (not suggesting trying a bowling ball but something smaller with an offset center perhaps a custom resin ball with an offset cast steel ball bearing inside)

[@mxz_archery]

Just realized you went from finally hitting 1M subscribers a few months ago to being on the verge of 2M right now. Well deserved! And as always great video :)

[@ShainAndrews]

Right.. because it matters.

[@william41017]

@@ShainAndrews Jesus! Op just congratulating him on his success. And, idk about Steve, but subs number matter for a lot of youtubers considering that's their livelihood.

[@eugenesaint1231]

Wow, this takes me back to the 50;s. We had a record player that didn't work (the audio didn't) but the turntable would spin. We'd put on a 33 LP and put a marble on it. I observed the same results as you did but had no understanding of the physics/math involved. Thanks for the excellent explanation, BTW, we would also roll up a paper cone, stick a straight pin through the pointy end, and hold that on a record to listen to our tunes. Way cool. Just sane... :^) Saint

[@richardchambers256]

Me too but I used a CAT. 😆

[@eugenesaint1231]

@@richardchambers256 Ha! 7 to 2 ratio, 7+2=9 :lives. Coincidence? I think not. He he he...

[@alphonsbretagne8468]

I did this when I was young as well. The paper needle trick works with a flat paper or cardboard already but slightly quieter. Though the best thing always has been to choose the wrong speed 😅

[@jhawkins4412]

Dito

[@jpsned]

My little sister and I used to use an empty dog biscuit box and a needle to listen. 😀

[@jameslimary755]

Very good at cutting out the technicality and still keeping the explanation satisfactory

[@Wayne_Robinson]

It's always delightful when a simple experiment reveals unexpected behavior.

[@omgitguy]

Seeing you advertise "The Länd" took me by surprise. I didn't know we were advertising internationally. Also, your pronounciation of "Baden Wörttembörg" is really adorable. 😄

[@linkerganove5756]

Nett hier aber waren sie schon mal in Baden-Württemberg?

[@niklasfischer7866]

LOL was about to stop watching the video but now I’m gonna keep watching

[@7cle]

´been listening to this half asleep and it’s so brilliantly narrated that I’ve recorded the video in the watch later list. Steve is one of the very few best science sources on youtube and better than very large institutions that allocate large sums of money into it. Steve has the knack of finding sufficiently mundane stuff that anyone can relate to, yet is scientifically relevant and catchy. Steve, you’re the boss. Thanks a metric ton.

[@gauravmitra150]

I would like to see an Eulers disk spin on a spinning turntable. Once when the disk and the turntable both rotate in the same direction (e.g. clockwise) and once when one rotates clockwise while the other rotates anti-clockwise.

[@philb4462]

@Nicegram_SteveMould001 You are a scammer. Messages like these are cropping up all over KZbin.

[@Willy-the-Fire-Putter-Outter]

The people have spoken!! These are our demands, our balls are in your court!

[@CreamusG]

I love that at 3:20 we can see the rolling shutter make the line curved

[@Duplicitousthoughtformentity]

Steve Mould is the perfect example of how to make science interesting and engaging for the laymen and encourage curiosity. Asking questions about even the most mundane of observations and interactions is invaluable. We don’t know what we don’t know until we ask why and how.

[@MisterIkkus]

I like thinking about your videos like they're a published study in a scientific journal. Love the idea of the conclusion of a paper being, "Balls and other round things behave weirdly on turntables."

[@YassFuentes]

As a physicist, this video is pure joy. Thanks for making this video available, Steve ❤️

[@kristjanpeil.bsky.social]

7:08 - so... when i pick up an opaque ball, and shake and twist it in the right ways, I can tell whether it's solid or hollow?

[@rhyskadekawa1763]

Did you make or buy that motorized turntable? Just typing in turntable gives the DJ turntables and I would love to have one for my classroom. Awesome videos. I embed many them as supplemental videos to watch for high school physics.

[@yeti9634]

He is using a pottery wheel. Check with the art department?

[@SerifSansSerif]

yeah, that's totally a potter's wheel...

[@forestschoenrock3040]

Lol

[@MrSkypelessons]

I have always found spinning things and reference frames fascinating, and as an English teacher, I find it fascinating that these very real forces are referred to as fictitious forces and pseudo forces and imaginary forces. I suppose it is to separate them from contact forces and EM forces, but it does seem odd to me. I read that even gravity is a fictitious force. If there's one thing we can be certain of, it's the force sticking our bodies to the floor. Very interesting video and very clearly explained. Thanks.

[@HouD]

Fictitious forces are not real forces. They do not actually exist like any of the fundamental forces (e.g. electromagnetic or gravitational), but they are very much necessary in order for someone in a non-inertial (i.e. accelerating) reference frame to explain what they are experiencing. As an example: you are a passenger in a remote-controlled car that is moving at a constant speed in a straight line. The car has no windows and has absolutely perfect wheels, shock absorbers, etc. such that you cannot even tell you are moving. Unfortunately, you are also not wearing your seatbelt. All of a sudden, someone turns the wheels sharply to the right via remote control. You will go flying into the left side of the car. From your perspective inside of the vehicle, you have no way of knowing that someone has caused your car to turn. From your perspective, there was suddenly a violent force pulling you into the side of the vehicle. What force has caused this experience for you? In reality, there was none, however from your perspective it must have been so, and so we would attribute (from your perspective) a fictitious force called the centrifugal force. Hope this helps!

[@MrSkypelessons]

@@HouD I do appreciate the answer, but try saying 'in reality, there was NO force' to someone who was badly injured or even killed by such a force. Surely there was such a force, and not just from his perspective - from our perspective too. And why have I read that gravity is also 'a fictitious force'? Is it because relativity claims (and I certainly can't understand the logic) that gravity is just a curvature of space and time? I have no idea!

[@empathogen75]

The difference between fundamental and fictitious forces is that if you change your perspective on a fictitious force, you can make it go away (in the sense that they’re no longer terms in any equation), and things like electromagnetism will exist in every reference frame. From an English major point of view - picture something like Fight Club. Tyler Durden was very real for some of the characters’ perspectives and even produced real outcomes for them, but was still not real.

[@HouD]

@@MrSkypelessons don't conflate the force that might injure someone (e.g. the contact force as they slam into the door of the car) with the perceived force that attracted them towards the door in the first place. We would not label gravity as a fictitious force. However, yes, in a very thereotical sense gravity is not a force like a push or a pull. However this has to do with General Relativity and advanced ideas in physics. For your (and my) every day life experiences, gravity is very much a force that pulls massive object towards one another.

[@MrSkypelessons]

@@HouD 'We would not label gravity as a fictitious force' - who is we? 'This led Albert Einstein to wonder whether gravity was a fictitious force as well. He noted that a freefalling observer in a closed box would not be able to detect the force of gravity; hence, freefalling reference frames are equivalent to an inertial reference frame (the equivalence principle). Following up on this insight, Einstein formulated a theory with gravity as a fictitious force and attributed the apparent acceleration of gravity to the curvature of spacetime. This idea underlies Einstein's theory of general relativity.'

[@thejeffstreet]

I really liked how the equations are presented here. Must have been a ton of work. Thanks for making the effort.

[@gregvondare]

Sounds like a way to understand precession and LaGrange Points. There's something deep about gravity and dynamics in this demo, but I can't quite crystalize it. Any ideas about that, Steve?

[@PeterNerlich]

I believe without doubt that there is that fixed ratio because the other factors cancel out, but that being a different constant for hollow balls is really weird. At first glance, it makes sense, but what if you consider the thickness of the ball "shell" as a fraction of its radius? Now you cannot discretely differentiate between whether a ball is hollow or not (or rather, solid balls become a special case of hollow ones), because at the limit→1 they are basically the same

[@Geerice]

if I'm understanding your comment right, basically the ratio for that hollow ball isn't exact. The ratio is for a theoretical ball with infinitely thin walls. A more complex equation exists for a solid ball with a cavity.

[@jameskerns717]

As you change the distribution of the mass by hollowing out the middle you make a gradual transformation from solid to a "thin shell". Of course, where do you draw the line at "thin"? When the assumption of a "thin" shell is good enough.

[@FadeEthereal]

Can you explain at what point does a hollow ball become equivalent to a solid ball? Is the change from 5/2 to 7/2 immediate?

[@arburo1]

As a 70 year old chartered engineer I am always learning new things. I had never seen this before, so thank you.

[@fotwen]

I'm so glad you mentioned something about coriolis. That's kinda where my mind went watching the ball go from closer and further to the point of rotation.

[@feedingravens]

Not only that - when I see the ball orbiting around a point, it reminds me of a moon. Almost as if it does not even need a planet to do such movements, or, all the planet does is give the cause for the orbiting moon to orbit another center.

[@dustinkirk3920]

@@feedingravens except, the moon isn't rotating on its own axis is it? Planetary satellites orbit their hosts regardless of their own spins. Hmm...

[@truongtran-sl6rh]

ok

[@fotwen]

@@truongtran-sl6rh alrighty then

[@fotwen]

@@feedingravens idk... Remember, the center of orbit between moon and earth isn't the center of earth.. so their orbit is a relationship between the two. The reason it's tidal locked with earth is also that reason. Adding friction to rotating bodies it completely different than orbital mechanics. As I know of. That ball is experiencing different tangential speeds as it moves closer and further away from the axis. Orbits do have a sling shot kinda effect. But I see that different than this. But I could be wrong.

[@seanmostert4213]

1:28 if you count the table orbits, and observe the ball orbits as shown … for every three ball orbits the table orbits a little more than three, and this appears to follow the number of Pi (3.14159) visually speaking, it would be great if Steve or someone could verify this with math.

[@RajivKumar-xp9qn]

No brother, ratio should be exact 7/2 according to calculations of moment of inertia... Here we are prolly seeing some human error or maybe the ball isn't really uniform... Also here we don't have the ideal case of exactly one point of contact, slight deformation occurs... It's just that our ideal physics book calculations differ from real world because we make many assumptions for simplicity... Otherwise in a completely ideal case, it isn't gonna be 3.14

[@ThalassTKynn]

The circular motion of the ball reminds me of the motion of an object in space orbiting a planet. If its orbit isn't circular it gains velocity as it decends, and then exchanges that velocity for altitude after periapsis. I have probably played too much KSP.

[@danwood1121]

I wonder if this is related to how the James Webb space telescope has that unique orbit?

[@MattStryker]

Steam says I've played KSP for 3,310 hours. And yet, when I get home tonight from work, guess what program I'm gonna fire up...

[@NateTheScot]

He has a video on that exact thing. Or at least I've definitely watched a video explaining why the orbit of planets and bodies are all evenly balanced because of oval orbits since it's extremely rare for an actually perfectly circular orbit due to universal gravitational pulls. This results in all orvits eventually balancing at a certain point in the oval which can lead to interesting alignments on very rare occasions which I'm fairly sure was him since I don't watch many other channels like this but I could be wrong..

[@Aashishkebab]

Unrelated

[@RichardHowells1234]

Fink dat so

[@hesido]

The 7/2 vs 5/2 between the solid and hollow ball is very curious to me. So balls with different thickness of shell would turn anywhere between 7/2 and 5/2? This hasn't been mentioned in the video.

[@vsm1456]

Yes. Someone in the comments said that if a ball has a hole in the middle, the size of 38% of its diameter, the ball would have Pi rotation period.

[@95greenbug]

What about different accelerations? A slow acceptation and a fast acceleration?.. then what about a cyclical speed? Increasing then decreasing speed? Would the ball stay on longer then?

[@xvr_demi_trees2937]

As a 2,055 year old Carpenter it amazes me that after all these years, we still love playing with balls!

[@jcf7401]

This is beautiful. Thanks for sharing

[@anoninunen]

I've seen this done with an umbrella as a bit of a parlor trick - good to understand how it works

[@Loctorak]

That IS a cool parlor trick!

[@andygodwin5062]

Ok dude, you've grown on me and I love these videos. Thanks for getting on with it and having the balls to just make the videos!

[@playr4]

There's this very hands on science museum in the SF bay area called the Exploratorium, my favorite museum as both a kid and an adult. They had an exhibit about this which was my favorite thing there. It was a giant turntable, like 4 feet-ish in diameter and a ton of discs and balls of different weights and sizes and some of the discs had holes in different orientations. I could easily spend my entire day there just playing around with the physics of it, trying so hard to get one to stay on for as long as possible.

[@DavidLindes]

was going to mention exactly this, but you beat me to it. :) I've spent lots of time there, too... they also have little sticks that one can slide through the holes in some of the discs, or also some rings, to allow them to be held still to set up. Another fun thing is rolling something onto the turntable from the side, as the turntable is basically an inlay on a larger table surface, and the heights are flush to each other... so you can roll a ball on at different angles, and see what it does as it transfers from the static table surface to the dynamic turntable surface. Much fun! Highly recommended to anyone in/around (or visiting) SF.

[@JimC]

I lived near SF from around 1980 to 1991. That was when the Exploratorium was still in the Palace of Fine Arts. I became a member and visited often. This exhibit was originally intended to show how something on the turntable would be flung off tangentially to it, not straight from the center. But the objects were small disks, and everyone found seeing them roll on the turntable was much more interesting. Getting pennies to roll was fun, too. I don't remember the museum having spheres of any kind for the turntable. I never thought of bringing a ping-pong ball. The most interesting object I saw there was one of those rubber feet for folding chairs or canes. It's shaped like a cone with some of the pointy end cut off, and the bottom was somewhat rounded. A rubber sink/bathtub plug could've be interesting to watch, too.

[@playr4]

@@JimC Aw I miss that location. It was so much more open than it's current place. You walk through the doors and just as far as you can see there were experiments and things to play with. It's a lot more moderny now, which is fine but it did lose that sense of awe it used to have. I used to go to summer camp there when I was a kid and we would have an hour before it opened with all the exhibits turned on to play around with. Easily the best summer camp experience I can remember.

[@JimC]

@@playr4 Sounds like lots of fun! In Chicago, back in the 60s, when I was around 11, my older brother worked on Saturdays in the Museum of Science and Industry for a time. He worked in the Swift exhibit (hatching chicks, tending a sheep or two and a few baby pigs). I'd usually go with him (if I didn't oversleep :) ). I couldn't go into the museum proper until opening time. But then I'd hightail it to exhibits that were usually crowded later in the day and wander around until closing. I loved it! The Museum has changed a *lot* since then, but if you're ever in Chicago go see it!

[@DavidLindes]

@@JimC interesting about the intent. I don't recall what their signage says on it these days, but I know they've had billiard balls among the other options. The rubber feet sound interesting!

[@undisabled1552]

Watching the ball on the turntable is mesmerizing!

[@davidosborne1306]

I'd like to replicate this experiment for demonstration purposes. I have two questions. 1) Do you have a recommendation for what to use for the surface of the turntable? I feel like it should be the highest friction coefficient I can come up with to limit the slippage, but needs to be rigid, so there is no (appreciable) deflection of the surface from the weight of the ball, and very smooth, so the ball doesn't bounce. As I write this and think about it, I think a good single piece of hard rubber would be ideal, as long as you don't put a bowling ball on it, but that's not something I can obtain easily. My thoughts on what I have available, would be a sheet metal disc, and coat it with polyurethane to increase both the coefficient of friction, and the smoothness, but I would appreciate your thoughts. 2) What RPM were you running your table at? I understand that the speed of the turntable does not matter to replicate the effect, but I'm pretty sure you didn't drive your turntable at 1700 rpm straight off an AC motor... lol. I'm wondering what kind of RPM range to target, so that I don't send a projectile flying off to hurt someone. I have access to a small variety of motors to choose from, I think I'll probably use a little 18v battery operated motor from a drill, but I'll have to put in some amount of reduction.

[@tinalisapattern]

You keep fascinating us as well as entertaining. Thank you for that Steve. And congrats to those, who choose you to promote "THELÄND". Perfect choice. I am from Germany and have seen the clip before. And yes, I believe for tech or science aspiring people, that is a perfect place to go to.

[@Norsilca]

Yeah I loved that his sponsor was a state of Germany!

[@nepoxx]

Around 3:10, as the image is frozen, we can see that the line appears to be curved. I expected this artifact on one side, but not both. Any ideas why the line appears to be curved this way?

[@theundercoverguy]

This is called rolling shutter effect, you can look that up on youtube or elsewhere and find lots of good explanations on the topic. In short, the reason that happens is because most cameras don’t capture image instantly, rather they sort of scan the frame from top to bottom. By the time the lower part of image gets scanned, anything in motion moves away from the earlier position and gets distorted.

[@nepoxx]

@@theundercoverguy Ah right, that makes sense. I expected the distortion to follow the direction of motion, but I didn't consider the shutter going from top to bottom. Thanks =]

[@soanywaysillstartedblastin2797]

Watching this after taking physics and this actually makes so much sense. This is actually very similar to rolling an object down a ramp, and that’s where the 7/2 ratio comes from The moment of inertia times the lever arm.

[@johnsomerset1510]

I guess you failed physics then.

[@soanywaysillstartedblastin2797]

@@johnsomerset1510 got a c-

[@dwdei8815]

So strange how our intuition of where the ball goes kinda depends on the ball realising it's on a turntable and behaving accordingly. I loved that point of your explanation.

[@HartenDylan]

Hi Steve, there's lots of fascinating ideas here that would be cool to explore. For example, if the spinning surface was curved like a Euler's disk, how might the motion differ compared to that of the flat or convex surfaces? Here you present flat, and earth is our convex. This harkens back to your "this should slip off but it doesn't" video with spinning concave and convex surfaces and a spinning band. Love the videos and making us foster unique concept connections!

[@kylelai7699]

Can we use Coriolis force to directly model the system in the rotating frame? That would be a bit easier ?

[@grahamdunning]

As someone who's been putting marbles and ping-pong balls on turntables in the name of Sound Art for the last 10 years, this video was very useful.

[@andregon4366]

What are those sounds used for?

[@grahamdunning]

@@andregon4366 I use ping-pong balls to semi-randomly trigger synth sounds as part of a music performance: kzbin.info/www/bejne/laGTdHqFpLNqgJI

[@insidejazzguitar8112]

Love this one. Glad you mentioned the Coriolis effect. When you showed the gyroscope, I thought you were going to talk about procession. That would be a good concept introduce here too. By the way, your videos are great! Adam

[@qovro]

You've got me visualizing a parade of gyroscopes dressed in fancy costumes.

[@suighrafa]

Dude, the math explaining the 7/2 was so beautiful. It made me smile!

[@TinSandwichUK]

As the ball gets up to maximum speed, doesn't it take up the inertia properties of a gyroscope especially when you nudge it of axis?

[@dylanparker130]

Brilliant stuff, as ever - loved finding out where the 7:2 ratio came from!

[@lucascobrea5445]

As someone who is really into frisbee golf and also into physics I can confirm that there is correlation with the gyroscope and the tilted turntable. The reason for the gyroscope moving is because as you push it your input travels around the gyroscope with the rotation

[@DrDeuteron]

For the "stationary" ball, as viewed in the frame rotating with table, it feels a centrifugal force of mv^2/r and a Coriolis force of -mv(omega) = -mv(v/r) = -mv^2/r. So the sum is zero.

[@lopezguy9587]

Id like to learn math like this

[@TheSparkLabs]

@@lopezguy9587 this kind of stuff is called kinematics! Specifically, this heavily relies on angular motion.

[@louf7178]

@@TheSparkLabs Would kinetics be more accurate?

[@DrDeuteron]

@@lopezguy9587 me to. Can you spot the mistake?

[@monika.alt197]

@@lopezguy9587 read up on rigid body dynamics

[@_dx_dy]

Gyroscope behavior as an analog to balls on a turntable is a really cool example of symmetries in physics :D (or at least perceived symmetries, but even those are helpful in advancing our knowledge).

[@honeybadger036]

04:00 I don't know about all these vectors, but the behaviour still makes perfect sense. As you say, the ball initially stays in place because there is an equilibrium between the speed of the balls rotation and the speed of the discs rotation (at that point/distance from the centre of the disc) Nudging the ball, and pushing it to a lower speed of rotation (closer to the centre of the disc) causes the ball to accelerate forward in a straight line (relative to the disc), because the ball still has its own speed of rotation, which is now faster than the discs at the point closer to the centre of the disc. But because it accelerates forward in a straight line, it forces itself back outward away from the centre of the disc, to a point where the speed of the disc is now faster than the balls rotation. As the ball reaches the outer edges of the disc where the balls rotation relative to the discs rotation is slower, then the discs rotation starts to push back against the balls outward acceleration. But obviously these 2 countering aspects are happening simultaneously. If the mini orbit of the ball were to be drawn, and dissected, so that the line of dissection is perpendicular to discs radius, then the inner portion of the mini orbit, (that half which is closer to the discs centre) would represent the half of the mini orbit where the balls rotation exceeds the discs rotational speed. The other half, would naturally represent the half where the discs rotation exceeds the balls rotational speed. If one were to leave the ball and not nudge it, I believe that after a very very long time, it would eventually fly off the table too.

[@telotawa]

0:18 are those orbits elliptical?

[@All-in-one-wq9nv]

I think so yeah

[@mpag6195]

it would be interesting to see the motion of the ball from a relative point of view with a 360 camera mounted in the centre of the turn table.

[@adebirkful]

I would imagine that the slipping which causes the spiraling motion (reference at 4:59) has more to do with friction from the surface rather than air resistance. The changes to the resultant velocity of the ball requires the surface of the table to apply a force. The only option the surface has to apply force is friction, which comes at an angle to the axis of the spin as the ball changes its radial position. This would cause slippage, which would transfer some of the kinetic energy of the ball, causing gradual changes to the motion. The fact that the motion changes when performed on the record is the strongest evidence for this hypothesis. The coefficient of friction is much lower between the record and the ball, so the force is much smaller, and can't remove as much energy per second, thus the procession of the ball's motion is more gradual. If it was due to air resistance, you should see similar behavior regardless of the surface.

[@Burgher1605]

0:52 thanks for the ball to hand size clarification here. 🤣

[@mattybrunolucaszeneresalas9072]

They are not normally that size?

[@anmolaryan3575]

While Veritasium's 'sliding finger under cane' video became popular that it ended up being asked in JEE Advanced 2020 ( I know there were a few research papers on it beforehand) This video has the topic which is favourite of professors at IITs for framing questions... I m gonna take a note 😁 Thanks Steve

[@subhadityanath4326]

Which video, may I ask?

[@anmolaryan3575]

@@subhadityanath4326 kzbin.info/www/bejne/oHqwmpumedJ1prs Veritasium explains here the sliding finger under cane phenomenon ( why always both fingers end up right below C.O.M.) And kind of same question was asked in JEE Adv 2020

[@fragglet]

When I saw this I was reminded of Lagrange points and how objects near them have stable orbits around them even though there's nothing there. Of course it's the same - another example of the Coriolis force. Great video, thanks!

[@kayasper6081]

Fascinating stuff, very well framed in images and clearly explained. Thanks for your video!

[@volzutansmeig7323]

Nett hier. Aber waren sie schon mal in Baden-Würtemberg?

[@brickmastere5535]

I’m curious, would a gömböc shape act on the turntable?

[@haidershaharyar]

I'm glad to have found you... purely because your videos make me remember my curiosity as a kid. The things you lose when life takes over! Great going!!

[@bekaemery2918]

This was such an interesting video. I don't remember any of my alevel physics but I do love stuff like this

[@ibblecabibble]

its because the ball has a 360 degree pivotal axis so it can spin at an angle close to the force applied. its sort of like putting a wheel on a treadmill depending on the friction the wheel could just spin and stay in place. the wheel can only spin around "one" degree of rotation but you throw in a ball that can alter its axis and degree of rotation 360 degrees the ball can pivot, roll, or spin close to the angle of the applied force. a ball is perfect for an object that has a variable axis degree or pivot point. (all of that to say the ball can spin in any direction so if its on a turntable it can spin with it and stay on)

[@betabenja]

3:20 you should not put a bendy line on your turntable. or use propellers where the blades are weird and floaty.

[@SteveMould]

I only had a bendy ruler

[@r0cketplumber]

@@SteveMould And that bendy ruler flexes for every rotation of the turntable, imagine that!

[@betabenja]

@@SteveMould if you have a bendy ruler, you need to turn the turntable while drawing the line with a bendy ruler. or walk around a stationary turntable while drawing the line with a bendy ruler. but not turning the turntable and walking around the turntable while drawing the line with a bendy ruler. you can, however, just walk round the turn table while turning the turntable but in the end, you will just have a turntable. and you might feel dizzy afterwards and fall over the bendy rule which you were not using.

[@ntwadumela_]

Once the ball is up to speed and static inertia is zero the ball's contact point however small is also being pulling inward on it due to the rotational drag, call it sidespin if you will. So the spin speed, speed of surface rotation , surface friction is relevant to counter/ induce centrifugal forces.

[@danpatterson8009]

Trying not to imagine this popping up on a kinematics exam.

[@BenHur872]

Your 'paradox' videos are my favourite ones, Steve

[@janrobinkautz742]

Nett hier. Aber waren Sie schon mal in Baden-Württemberg?

[@dopehr]

at 1:44 you said no matter how big/heavy the ball is; However, in the equation "radius" and "mass" were pointed 7:04 . My question is why we had I=5/2(mR^2) in 3 tests out of 4 test ( pool ball and the metallic balls)? was that coincident or are we claiming that any solid ball has (mR^2)/I equal to 5/2?

[@erwinvangrinsven9345]

For more precise rotations, please use a Technics SL1200, thank you.

[@malnoir8054]

Nett hier. Aber waren Sie schon mal in Barden-Württemberg?

[@slash_me]

Came here to find this comment. This sentence is everywhere and now it's in this comment section.

[@MikeV8652]

You explain things so much better than Veritasium, which I greatly appreciate.

[@ehtuanK]

Nett hier, aber waren Sie schonmal in Baden-Württemberg? I can't believe they sponsored you.