You can't bounce a ball under a table

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Thanks to Hugh Hunt for the idea for this video.
If you try to pass a bouncy ball under a table, if it hits the underside of the table it will just bounce back out the way it came.
Here's the golf ball paradox video: • The Golf Ball Paradox
Here's the turntable paradox video: • The Turntable Paradox
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[@SteveMould]

Does it make more sense now? Maybe. idk The sponsor is Incogni: The first 100 people to use code SCIENCE at the link below will get 60% off: incogni.com/science

[@ThereIsNoSp00n]

You could reduce friction between ball and surface by applying a lubricant or talcum powder, thus reducing or even cancelling the effect.

[@EquaTechnologies]

i will stay private with this, yah?

[@oznerolk]

it reminds me of Michael Moschen's "Energy" routine!

[@rogerrabt]

Let's try with a football/rugby ball? edit: Also what happens in 3 sides?

[@swannee69]

pool and snooker players understand this principle, the spin reverse the direction when it hits a 2nd cushion

[@JanStrojil]

Now explain why the children’s bouncy ball always gets stuck under the sofa and never bounces out.

[@bigboi1004]

It likes it under there

[@soccerkenshin]

The sofa is just a low table. Otherwise why would there be so many crumbs in the cushions?

[@priapulida]

osmosis

[@EvanPederson]

Gremlins

[@xplosiv3z799]

Electrolysis?

[@Vlow52]

That’s also related to the “impossible” shots in snooker and other billiard games. When cue ball suddenly reverses its direction due to the spin gripping the cloth or cushion.

[@cage989]

this is more like a ball coming off the rail with sidespin

[@Vlow52]

@@cage989 yes, that’s what I’m talking about. If you inspect a masse shots with a lot of spin, it’s essentially the same forces of friction that reverses the direction of the ball after gliding on a flat surface. It’s probably possible to replicated it in 3d by using slippery or magnetic surfaces that ball bounces off, but essentially you can see these effects in games like baseball and golf, when spinner ball runs on curve by catching air molecules.

[@MikkoRantalainen]

The "impossible" snooker shots also use the fact that there's a difference between static friction and sliding friction. And because change between those happen pretty abrupt when force remaining in the ball decreases (thanks to sliding friction), the ball seems to suddenly change behavior in the middle of the shot which gives the "impossible" appearance.

[@cage989]

​@@Vlow52 the cueball slides along the surface before coming back with backspin, it's not the same as a ball changing direction immediately when coming off a rail with side spin

[@trajectoryunown]

Does that include making the ball jump as well?

[@KEKW-lc4xi]

my procrastination game is strong today.

[@piotrek5s170]

same here

[@jasonspence]

Before getting caught up in the many-sides version, I think it's important to mention that part of the reason it's hard to bounce a ball under a table is that the rotation robs some of the horizontal velocity, making the ball hit the underside of the table much earlier than you think it should. You have to bounce the ball considerably farther away than half way under the table for it to come out the other side.

[@balares]

Basically what i was thinking, if you made sure it bounced after the middle of the table (maybe with an extra outwards rotation) it would most likely just pass through.

[@Skank_and_Gutterboy]

Yep. Where I was caught flat-footed is how often it does this effortlessly (I actually tried it). I figured that it would nearly always bounce under the table and across the room unless you put some back-spin on the ball. Nope!

[@luminescentlion]

I think the implication was that the ball would hit the bottom and top at least once.

[@luminescentlion]

​@@repentandbelieveinJesusChrist1 “Beware of practicing your righteousness before other people in order to be seen by them, for then you will have no reward from your Father who is in heaven." Matthew 6:1

[@Skank_and_Gutterboy]

@@repentandbelieveinJesusChrist1 🎶He's got the whole world, In his ass...🎶

[@holaferfi]

I love how well you keyed out your hand on that slo-mo shot, what an editor!

[@SteveMould]

Some recognition at last. Thank you

[@cryptfire3158]

i saw a blur.. had no idea it was his hand.

[@chocolateblocks]

He keyed out his hand? Wow! I never noticed it! It was simply too good.

[@freeEnd_]

Duh, he never keyed out his hand, its just a camera that records at 100000000000000 fps, so he just threw the ball and recorded it with that camera. He sayed that he keyed out his hand because he didn't want us to know that he has access to governments secret objects. Simple as that, he is the president of US

[@PinkeySuavo]

which second is that? oh nvm it was a joke xD

[@Lombardio]

"Low-poly cylinder" is my new favorite name for convex regular polygons. You brought up friction at the collisions, but I kept wondering if you were going to address deformation of the ball at each bounce. My initial thought is that the ball deforming when it hits a surface mostly just increases the surface area for friction to be applied, but I don't know enough about bouncy ball physics to decide if it is a crucial part of the dynamics or not.

[@Shademastermcc]

Everything is crucial if you're being precise enough.

[@ewthmatth]

You're definitely misusing the phrase "convex regular polygon" here.

[@Lombardio]

​@@ewthmatthYou're right. Convex regular polygon prism?

[@eben7248]

@@Lombardio Just "prism" would do fine.

[@stevenculver7228]

Deformation is crucial for two reasons: (1) forces need area to transfer between objects (i.e. stress), and (2) the total linear impulse is a function of the friction force and the duration of the impulse. If the ball was too stiff, the friction force wouldn't have enough time to change the linear momentum enough to bounce the ball out.

[@ClearAlera]

I play racquetball, which is basically a bouncy ball in a cube like this. I remember when I was first learning, the ball kept bouncing off surfaces in ways that seemed more extreme and counter intuitive. It's second nature now, but this video was great at explaining what my brain had trouble with all those years ago!

[@cengland0]

Came here to say this. This is the principle of my Z shot where I can make the ball go parallel to the back wall leaving only a couple inches between the ball and the back wall. If you play with an old ball or moist walls, the ball doesn't spin right and doesn't "Z" properly.

[@Songfugel]

This is so simple and logical, but I never even stopped to consider this might be happening when you throw a ball to bounce on several surfaces. Would be nice to see a similar experiment with a gyro-type of bouncy ball that tries to eliminate this change in spin

[@bluesight_]

one could maybe apply lubricant to the ball

[@pzsn5075]

​@@bluesight_❤

[@flyingdev]

I bet if you did this demonstration with a ball covered in different coloured spots you'd get a nice visual indication of the change in rotational axis after each collision. The various spots would move more or less depending on how close to the axis they are and after a bounce a different spot would stop moving

[@firytwig]

That would help immensely with the animated example since you really can’t see the spin because there’s nothing to track on the surface

[@dolphin64575]

I'm fairly certain soccer ball design on bouncy balls exist...

[@nicholasbyram296]

Zigzag stripes is best for viewing rotation on a sphere.

[@ThatOpalGuy]

​@@nicholasbyram296I was also thinking stripes. Doing the experiment with all 3 designs would be fun.

[@alexanderdickens2517]

Indeed, in table tennis there are balls made of two different colors to more easily teach students spin

[@aettic]

Steve, you are one of the best science communicators I've ever seen. So glad that you make these videos. Your kindness and endless curiosity are greatly appreciated.

[@huuuuuuuuuuuuuuuuuuuuuuuuuuuhn]

seriously one of the few channels where whenever I see a video being recommended I cannot resist. It also never feels like a waste of time watching them

[@jacobshirley3457]

I'd say Steve Mould and ScienceClic are equally good at converting complex topics into something intuitive.

[@me0101001000]

What I appreciate is that he communicates things in a manner which someone with no background in science can follow, but then an expert can also appreciate the content as well. I'm an aspiring professor of engineering, and Steve's delivery is something I've been paying close attention to so I can be a better teacher.

[@BloopTube]

He nails it on a level that only people like the curiosity show and Bill nye have matched,

[@anearforbaby]

Absolutely true. I have been really disappointed by some other channels losing sight of their purpose, but everything I've seen from here remains focused and thoughtful.

[@HunniebeeGames]

my cat really liked this video

[@peterpumpkineater6928]

Why is this the top comment

[@sneakmore]

its pretty amazing how much you can learn from watching things in slow motion. like you can say it to me and explain it super well but showing it to me in slow motion I actually can rap my head around the ball rotation and the effects that has on each bounce. played quite a bit of 8-ball in my day and I think that was my biggest hurdle was the massive difference in friction

[@ChrisTooley]

I like that you use props like a book and a box - it shows people that they can explore physics with the stuff they've already got in their house

[@ak47adultswim]

physics is everywhere and its fucking sick

[@tenv]

Look at this guy, bragging about his books and boxes

[@ThatOpalGuy]

So that scanning electron microscope I just bought on Amazon was a waste of cash?

[@ak47adultswim]

@@ThatOpalGuy nah man thats a certified pimp move my guy

[@adamcetinkent]

And what a book, eh?

[@MrKyle700]

Table Tennis, once you are past the "basement" level, is almost entirely built around this phenomonen. You can pretty easily learn how to bounce a table tennis ball back over the net using the spin alone. the idea that the collision changes the spin is essential to master. super cool video! i bet you could expand this by like filming the spin of a ping pong ball between highly skilled players

[@TraumaER]

Mind blown

[@rcbuggies57]

I was about to say, this is completely expected. We hit counter topspin balls with our rackets basically horizontal and the ball basically goes back on the exact same arc despite the angle of contact dictating the ball should go the complete opposite direction.

[@RavenMobile]

Interesting.

[@auckwads8169]

Funby i woukd say a tt ball does the opposite. Back spin serve is back spin firdt bounce server side and back spin still on subsequent bounces due to lack of friction. Long pips reverse the spin due to lack of friction and a direction change. counter looping uses the principle in this vid as your using grippy rubber

[@-danR]

As soon as I saw this video, it was the first thing that came to mind, having played over half a century ago with the particularly grippy, even tacky, new Butterfly surface. (I'm avoiding naming specific products). It was the most natural thing to be counterdriving almost directly forward with a blade that was almost parallel to the floor into a topspun ball traveling directly horizontally at the top of its arc, if that incoming ball had a high enough bounce.

[@sqarfuls8649]

Incredible video! I remember the golf ball video ages ago. While your communication skills really carry your conveyance of fairly complex ideas in an intuitive and digestible manner, this video was the key to understanding that video fully. Once I "got" this video, it instantly allowed me to understand your previous. Excellent work, as always! :)

[@TheBugkillah]

My three year nephew asked why the bouncy ball bounces so funny. I made him watch this video. He now calls me “Uncle Meany”.

[@BenRHarsh]

I just want to say, that shot where you show the ball spinning and floating slowly through the air was absolutely brilliant. I still have no idea how you pulled it off. I sat here for nearly 3 minutes (maybe like 2 minutes and 46 seconds?) trying to wrap my brain around it and couldn't figure it out! Bravo.

[@koriko88]

You just need a high-speed camera and someone to throw the ball in a few times until you get the shot you want.

[@ta1708]

​@@koriko88lol

[@Oscar-yk6ww]

@@koriko88it’s a joke.

[@IsStillVicarious]

I think he might have keyed his hand out or something? I'm not sure, but whatever he did makes it such a cool shot.

[@selimvergili7001]

which shot exactly?

[@Moingboy]

I would love to see a practical follow-up about putting an initial spin on the bouncy ball so that it continues out the other side of the table.

[@u1zha]

Is there a way to do that? The initial spin would just make the returning force (when it hits the table top) stronger. Unless you spin it so hard that it misses the table top already after the first bounce.

[@Aislinsweetdreams]

​@@u1zhaI've repeated the experiment with different balls, and got mixed results

[@KonstantinUb]

I also really want to know if this works

[@dielaughing73]

​@@tafazzi-on-discordthe high friction of the ball against most surfaces is the reason for all this behaviour. I'm halfway into the video and I'm a bit surprised he hasn't mentioned that yet

[@gonelucid]

Or just not bouncing it so hard. I feel like all these commenters are too wrapped up in science and havnt played any games or sports..

[@brianhartling7767]

The variables like the materials of the table, floor, and ball are key. If you have something like a ping pong ball and a surface that is slick, because you are reducing the contact friction, you will likely overcome this.

[@takuid]

Such a generalizing title, almost click bait. First time seeing the channel and not gonna stick around, no pun intended....

[@alexanderdickens2517]

Ping pong is interesting because you’re right: on a table, it does not change spin due to low angle of incidence. However, once it hits the racket, the effect in this video is almost the entire game. The ball is gripped by the paddle’s high-friction surface, and shoots back spinning in the opposite rotation.

[@dbsmash2]

Steve, I so appreciate how you can break something down to make it intuitive! Thank you for the service you provide, which is entertaining, while making something really cool makes sense to people :)

[@BrettDalton]

Interesting observation I had in that demo was it felt like the change of direction was happening on odd number faces. Wonder if it's conincide or there is some maths there. Would love to see the experiment with 3 faces and 5 faces.

[@Martian8]

My guess would be that each tube has a consistent and particular face that causes the change of direction. For the square it was the third face, for the octagon it actually looks like it’s the fourth face (but it’s hard to tell - it might be the third one). Certainly, for the first (2D) example it was the second face I imagine the designated face will probably be the one that corresponds with the (7*0.5)/2 oscillations in some way. My guess is that in order to complete a full vertical oscillation, you must complete (7^0.5)/2 horizontal oscillations. And so if you complete 1/4 of a vertical oscillation, i.e. entry to inflection, you complete (7^0.5)/8 horizontal oscillations. That comes out to about 0.331. So the ball will have to travel at least 33.1% of the way round before changing directions. For a square, the first face to be more than 33.1% of the way is the third face. For an octagon, it’s the fourth face. For the 2D example, it’s the second face.

[@mrWade101]

As someone who plays table tennis, this is very intuitive but incredible nonetheless.

[@lanzer22]

Exactly what I was thinking. Receiving a top spin always makes the ball want to bounce up, etc.

[@martinda7446]

Try it with a bouncy ball... You will do some ''impossible'' shots. I was wondering about the extreme lightweight smooth ping pong ball... I bet that will bounce under a table.

[@PajakTheBlind]

yeah, although this is somewhat "reversed" and "amplified" - it's the ball that is tacky - not the surfaces and due to higher inertia of the ball it does not die down as fast as table tennis ball

[@kikamonju]

I appreciate how much info you added that isn't just answering the prompt of the title. Also taking a limit as teh number of side approaches infinity is a cleaver way to figure out the relation between the under table bounce and the golf ball paradox.

[@imjustbryce6235]

Now explain why my wife left me

[@JonasThente-ji5xx]

Ouch

[@gentlemanthesavior9601]

Prob your wife is the ball, cuz she doesnt want to be with ya.

[@vilian9185]

skill issue

[@vilian9185]

​@@gentlemanthesavior9601the ball come back his wife didn't

[@digom4rs942]

Cause u keep watching YT videos instead of paying attention on her.

[@silviavalentine3812]

The part where you're moving the ball in slow motion in 3:00 just made me smile. I can just see the dedication in trying to show everyone how the dynamics work. 😊

[@Galactic_fart_sniffer]

But wouldn't that be wrong? The spin increases the angle of motion thus there is angular momentum and newtons third law states there is an equal opposite reaction. Idk everything so please Correct me if im wrong

[@TimeBucks]

Thanks for the awesome content!

[@user-di7dg3si3f]

This video is made to easy to understand so nice ❤

[@adijatbello]

Absolutely amazing

[@MDMarufIslam-yv3bi]

Nice

[@Thonga5]

now explain why bouncy balls are so tasty and why i feel ill

[@mike1024.]

I've watched several of your videos related to fun objects that you've made transparent, but I never realized how much physics you also do on your channel.

[@ExplodingDarth]

This made it so much easier to understand, specifically breaking down the table underside turning around and the square model, I absolutely love this!

[@sandoncrowder7839]

An effect I noticed years ago with a basketball is that if you spin it and bounce it, it will come up spinning the other way. I never put more thought into it but it's cool to see the changes that a little spin can make.

[@wayamy27narf]

I thought about basketball, too. This puts a better perspective on why the ball spins out of the rim so much.

[@jinsong6940]

Wow I remember watching your first video on the cyclinder and not quite being satisfied with my understanding of the physics but it totally makes sense when it's simplified down like this. I'm amazed.

[@XxS4NN4SxX]

What I love the most about this kind of videos is that they are no longer just science communication, they are also straight up science.

[@nerdvana2]

A thought experiment: One model for understanding light is that it consists of particles. Light shined on a surface always reflects away at the same angle it came in on. Presumably, if you shoot a light particle down at the floor under a table, it will exit with at the same angle and not bounce around. That's the "angle of incidence equals the angle of departure" rule. Given all this I wonder what size/diameter of minute particle is required, such that spin becomes a significant factor in the departure angle?

[@NoNamer123456789]

Quantum mechanics is where mechanics break down, to speak in simple terms. First of all, a photon has no rest mass. Secondly, it can't spin like a ball because it's got no dimension. Honestly, it's better to think of it in terms of a EM wave that due to the collapse of the wave function results in being quantised. That's also very simplistic, but it gives you a better analogy in this case. As for how small a physic object with rest mass can get... Realistically, you'd start having problems with surfaces (and atoms themselves) being uneven before this effect vanishes in the world of quantum mechanics.

[@nerdvana2]

@@NoNamer123456789 Thanks for the info. Makes sense. This effect seems also dependent on the actual incident angle, and the relative traction between the ball and flat surfaces. A tennis ball, for instance, seems to have less traction on a smooth surface than a rubber ball, so the surface doesn't impart much rotation to it. I can easily toss a tennis ball under my table at a high incidence angle, and it shoots right out the other side without "reversing." Still, an interesting phenomenon.

[@Iluvatar196]

Ah yes I concur. Definitely understand what u guys are talking about

[@Vexas345]

​@NoNamer123456789 Photons don't spin like balls, but they do have spin, and it does change when they get reflected because their polarization is changed. This also changes their reflection probabilities since the angle of polarization relative to the surface matters.

[@NoNamer123456789]

@@Vexas345 I'm gonna be honest and say that's mostly above my level of understanding, but shouldn't it matter which reflection you mean exactly? Like, if we take a perfect mirror, for instance shining a beam of light from water to air at an angle that causes total reflection (40 something degrees IIRC), does the polarisation/spin matter?

[@Cierbhal]

Steve, you're soo good at teaching. I appreciate this channel a lot.

[@jerrylive365]

Your videos are always informative and entertaining. Thank you!

[@Jon-cw8bb]

This makes the cylindre + ball so much more intuitive. I found the previous video hard to really grasp but now it seems obvious! This is really amazing stuff

[@jkc491]

Not sure how or if this applies (haven't watched the original golf ball paradox), but this reminds me of missing golf putts and basketballs touching the entirety of a rim but not scoring. awesome video! keep up the good work.

[@sventeunissen4321]

That is in fact the motivation for the original golf ball paradox! And (obviously) where it got its name.

[@johnnyfrankenstein0123]

Every time I miss I'm gonna shout golf ball paradox

[@swealf-nonofficial]

As an engineering undergrad student, this is very interesting to watch. I can apply my knowledge of dynamics onto your intuitive explanation and understand the topic even better. Thank you for posting always such interesting videos.

[@Dan-hs8lb]

this channel is really fantastic - what an amazing demonstration of thoughtful science

[@archaeologistify]

This topic is just far enough beyond intuition that it is fascinating to watch.

[@sukira_]

Bouncy balls have a PhD in hide and seek. Prove me wrong.

[@salerio61]

There was a paper published in Scientific America in about 1980/1981 that covered all of this including all the analysis you have done in this video. The author even gave some examples of him wanging a ball in a squash court and how it bounced off the walls and floor - he also stated that those experiments frightened him so much he would be reluctant to complete them

[@Hellrider_145]

I began this video thinking "easy, backspin", and then I was amazed that this somehow linked to the golf ball paradox. Great video!

[@LampDoesVideogame]

Your slo-mo at 1:05 is enough to see what's happening, to me. Ball's going a direction, which can be represented by down and right, ball hits the floor which transfers some of the forward momentum into rotational momentum, ball hits the ceiling transferring all that rotational momentum backward, and spinning it the other way, and then since it's spinning the same way it's going when it hits the floor again, it continues coming back. That's cool.

[@grahamcracker_wookie]

I love this. My 8yo and I mess around with bouncy balls in the house. And I had to explain if you hit the ceiling it'll come back. But I've always wanted to know the physics. The rotation and friction combination is just a cool physics interaction. We actually have some odd shaped bouncy balls, flat edges, egg shaped, etc... and how they make then the physics unpredictable.

[@grahamcracker_wookie]

@@Juggler534 Haha, our favorite one is shaped like an egg. It makes for fun games of trying to catch it. Also always run the high risk of breaking something in the house.

[@davidsnead7728]

You're really good at explaining things. Thanks for the interesting video.

[@DroneLearner]

Enjoyed your Christmas Lecture demo

[@Trepanation21]

Dude, I love this channel. It's _exactly_ what I enjoy about being a curious person. You present these fascinating and interesting snippets of SCIENCE! and the world into these moments I can engage with in my day, even though my life isn't particularly involved in these ideas. Love it. Thank you for all you do for us, your fellow curious people around the world.

[@benmcreynolds8581]

This is related a lot to your video of a ball rolling on a spinning table. It shows that the backspin effects each bounce due to friction & change of rotation. I used to play with the spin of bouncy balls growing up. It's facinating

[@samtibbitts]

I remember as a teenager analysing dribbling a basketball and noticing that I was not simply pushing the ball straight down to the floor but rather I was pushing it forward slightly and imparting a small backspin. I also noticed that the ball bounced back up toward me with a small forward spin.

[@nostradamusus]

Love your videos, as usual.

[@jajssblue]

Does this hold true for only convex polygons? I wonder how a star shape might behave.

[@colintroy7739]

+ i want to see where this thread goes

[@kevinstandart4310]

My guess would be yes. Even with progressively more acute angles within the shape (Circle > Octogon > Hexagon > Square > Triangle) it still works. So a star-shaped polygon would be like 5 triangles put together. A practical experiment could be difficult though.

[@TgWags69]

A very pointed question.

[@kittvulpin]

Well in a star shape there is definitely a higher probability of hitting a concave surface that goes against the general motion of the ball. If that happened, I would think that would change the motion of the ball entirely. The challenge would be to calculate how likely the ball is to hit one of those concave surfaces. I would think as long as it hits only convex surfaces it should still work though

[@operator8014]

Seems like it should work the same in theory, but I suspect it would be difficult to make a physical demonstration. Maybe a computerized simulation with perfectly elastic collisions would work out.

[@johnsoutherland3459]

This actually does a great explanation of the carnival basket game. Where you have to toss a ball into a basket angled on a platform and get the ball to stay inside the basket. Most people just try to toss the ball very slowly into the basket with no spin. But this explains no matter how slowly you toss it, the basket will impact a backspin that causes the ball to bounce out. But if you impart a small backspin on the ball when you toss it, it will be more likely to stay inside the basket by reversing the spin to forwards.

[@philip1382]

Oh man! I found this out 30+ years ago as a kid when I used to throw a bouncy ball onto a piano bench underneath a piano. For a while I couldn't tell if it was just bouncing off the piano and returning but it looked like the ball was just changing direction. Nice to have that memory validated.

[@kvg4790]

This is used in pool/billiards. Side English at a rail and you change the angle the cue ball bounces off.

[@raedev]

What I think surprises me the most of this is that this seems to be caused by the spin of the ball and friction pushing it back, but that tiny bit of spin seems to out-do the energy of the bounces. That's what makes it feel strange to me. It becomes less strange if i think of the examples with more angles, as the bounces there are tinier and "waste" less energy, but seeing it on just two parallel walls, my brain just refuses to accept it lol

[@mysock351C]

In this case its due to the fact that they have enormous coefficients of friction when new. The surface is just _really_ sticky, and combined with the high coefficient of restitution it always made playing with them interesting, at least at the start. The first couple of times before the ball gets dirty, it does interesting things since it picks up so much angular velocity when it collides with a wall or the floor (Edit: and it becomes less strange perhaps when we realize that the opposing side of the ball has the opposite linear velocity when it contacts the next surface). After that, it just bounces the way you think it would due to the surface no longer being as tacky after it picks up a layer of dirt. Clean it, and its back to its old self, again.🙂

[@woob31]

I agree with your comment, intuitively it does feel strange. The way I tend to explain it is I believe one thing @stevemould could dig a bit further is the relative speed of the surfaces on impact. With the rotation speed gained on the first impact, the relative speed of the back spinning ball and surface during the second impact is approximately 2x the horizontal speed of the ball itself. That's how it can go back, not only cancel it's horizontal speed

[@mysock351C]

@@woob31 It would have been instructive to see him actually put some oil or talc on the balls surface. The real “solution” to this paradox is that when the ball collides, a substantial portion of its linear momentum is converted over to angular momentum due to the no-slip condition at impact. This slows its horizontal speed component quite a bit, and stores the needed energy to reverse its direction as angular velocity. You could even solve it analytically since it’s a fairly simple problem. Eliminate friction, tho, and it would just sail out the other side.

[@mysock351C]

@@woob31 Now, what would be interesting to see is what percentage of the balls linear velocity is converted to angular velocity. Its possible that there are internal mechanisms in the material itself that also convert a portion of the velocity that is perpendicular to the table's surface to angular velocity as well. That would potentially account for why it doesn't quite intuitively do what we expect it to.

[@TheRandomYoYo]

I feel happy that my dynamics intuition was correct. It was the Spin and the interaction of that spin with the surface that creates that path. Love that a simplification exists for this problem!

[@ohvizn6119]

Ive always noticed and loved this effect when i was younger its nice to see a video on it

[@lfoggy3061]

Really interesting Steve, thanks. What happens if you reduce the friction between ball and surface? Your rubber ball is very sticky with lots of friction. A steel ball on a hard surface will have much less friction. Or what about a hypothetical zero friction ball/surface interaction. If there is no backspin then the behaviour must completely change?

[@TacticusPrime]

Without friction it would bounce but never catch, so it would definitely pass right through.

[@TheSilverShadow17]

If there were no friction the ball would keep bouncing indefinitely, but would also continue to speed up in the process from the momentum gain.

[@dielaughing73]

This phenomenon relies entirely on a high-friction interaction. As does all the weird behaviour of bouncy balls (apart from their sheer bounciness)

[@TheReallavaman281]

​@@TheSilverShadow17 No. Momentum is always conserved in an isolated system (frictionless), so it would absolutely not speed up. This is a pure extension of Newton's 3rd Law.

[@killingtimeitself]

you can also think of the balls impact as the angle of the impact, imparting a potential energy on the ball, in a vector related to the angle. But since that potential energy takes time to store and release, by the time it's converted into kinetic energy, the ball has moved (in this case rolled) to a different position, which changes the vector of the kinetic energy in this case, releasing it in a different angle. I find that to be a little bit more inherently intuitive.

[@Lillbear3]

But wouldn't that require that all materials do the kinetic energy conversation at the same rate? Otherwise the angular change would be material dependent.

[@killingtimeitself]

there are definitely technicalities here, like the basic fact that something bouncing at an angle is going to bounce back with the opposing angle, barring externalities. but just for understanding this specific thing, i think it makes a point. Materials are definitely a consideration but given the squishy nature of a bouncy ball and the not so squishy (in comparison) nature of something like fiberboard, it's not super significant. I suppose some high speed footage of a bouncy ball would be rather amusing wouldn't it? Though it would change depending on the material of the ball itself. @@Lillbear3

[@notsofresh8563]

@@killingtimeitself If a rigid metal ball with the same mass and coefficient of friction as the bouncy ball is used, what effect does it have on the spin and release angle?

[@killingtimeitself]

much less considering the metal wouldn't squish nearly as much for as long, meaning it would be much less pronounced. @@notsofresh8563

[@Lillbear3]

@@killingtimeitself that is a reasonable resonse. Your model does help thinking about whats happening, and I agree that material differnces would be minimal. A high speed video close up of each of the bounces would be cool.

[@canbakr5602]

One of the best youtube channel of all time. Love you Steve!

[@DrRusty5]

As always perfectly explained for any audience. I can't help but feel you are going to one day inadvertently solve one of the great mathematical challenges, such as the Navier-Stokes existence and smoothness problem.

[@dhayes5143]

He'll demonstrate the Banach-Tarski paradox in real life.

[@firaasantar4503]

I noticed something similar if you take a basketball and let it fall while it has rotation (rotation axis is perpendicular to the floor) it will bounce back spinning in the opposite direction.

[@georgeisenbarg8302]

your transition to data brokers was so smooth i was waiting for the tie back to the video... have ever sold inssurance? very good video mr. mould up until i felt as if you were manipulating me into buying inssurance... love your videos sir!

[@daviddavidson3639]

Breaking down the physical components impact to impact (the spin and velocity of the ball) really does help highlight what is happening as we increase the number of impacts/collisions occurring. Reducing to a simplified state sometimes just allows us to understand what is happening and we can build on it like towards the golf ball paradox.

[@methamphetamelon]

I already had a hunch it had to do with spin. Same reason a rubber ball spun backwards and dropped will bounce backward, but now have forward spin and bounce forward, then getting backspin, etc. etc. Literally hops back and forth until all the energy is lost. Used to do it all the time as a kid. Love spinning a bouncy ball different ways to see how much of an effect it would have when bounced off various surfaces.

[@pauls5745]

Interesting! Compare now to a steel ball bearing or polished rock, to see if elasticity and friction have some effect. Might be neat to look into what trajectory and velocity effects are.

[@Boodoo4You]

You definitely will not see the same behaviour. This only works because the bouncy ball has such high friction that it hits the top, grips, and bounces back. Without that initial grip it will keep its forward momentum.

[@davemuso1958]

yeah Steve, oil up your table!

[@jamoecw]

the friction is key. any kid who has played with balls around tables knows that one can bounce a ball under a table, and the only way he could get the result he did is due to the type of ball used, which as he stated tends to have an odd second bounce. use any normal ball like a tennis ball, basketball, etc. with more consistent bounces and you will have trouble replicating his result. this is why the title works in getting clicks, as many people know he is wrong and are confused and want to understand what exactly he is talking about. one should also keep in mind that the table and ground surface also matter, though to a lesser degree.

[@bumbo222]

This video also reminds me of the most recent Veritasium video about the SAT question that everyone got wrong. I think the weird oscillating behavior of the ball in a container could be related to the coin rotation paradox. Also, is the motion caused by the friction between the ball and container generating a perpendicular force as a result of gyroscopic procession?

[@compugab]

Great explanation. It made sense thanks 😊

[@BritishBeachcomber]

*Steve Mould...* One of the best science explainers on KZbin. And non of the "I am always right" attitude of *the notorious Derek.*

[@bren8511]

A fellow schizophrenic out in the wild

[@ruzziasht349]

Non or none? know or is no the difference?

[@silvenshadow]

I love this video, but at the same time it makes me feel like a weirdo. Lol. Thanks for the awesome content!

[@dragunityx12]

Gyro Zeppelli and Johnny Joestar be like:

[@iarmycombo5659]

This deserves a short. I was familiar with bouncy balls doing that (throwing it between floor and ceiling in my home lol) and i knew it was because of spin, and i knew why bouncy balls changed their spin, as i had played table tennis with a normal ball and a bouncy ball because fun. And if you dont know spin is a great deal in table tennis just like in actual tennis. Any ball game really. Im saying this because the ball in the tube made no sense to me. But after you showed me how this phenomenon that i already actually did know and have an intuition for through all my time playing the sports and just fooling around with them, it makes perfect intuitional sense right off the bat. Even though the actual physical forces were kind of vague, now i actually feel like through that intuition i could at least make a guess to what they could be. And that happened very fast. Which is why i think thia deserves a short, cuz it can be such a cool message and it can actually be received.

[@troypollonais9143]

This video, and to a greater degree, this channel, is the definition of KZbin rabbit hole

[@7chanconn7]

If you control the initial spin of the ball, can you get wildly different results? Everything you talked about assumed the ball had no initial spin, but I'm curios how that can affect things.

[@Mueller3D]

For the table bounce, as long as the ball bounces off both the bottom and the top, I think the result should be basically the same. Obviously with lots of forward spin, the ball might just bounce off the bottom and miss the top on the way out, and with lots of back spin could reverse and never hit the top as it comes back. But as long as the ball hits two opposite sides, at least one of the bounces will have back-spin.

[@wbfaulk]

At 7:57, I'm assuming that's a Mould hexagon you're talking about: the sort with eight sides.

[@RavenMobile]

That's pretty funny! 😆

[@sjchew1539]

Is this a reference to Parker Square 😂

[@user-rm2qj2jh4l]

That is so interesting, great video! I love how Steve goes from talking about simulations of charged particle motion to native advertising so fast you almost think that those pesky spam emails and data brokers actually have something to do with the rest of the video 😂😆 I see you, Steve, I see you! haha

[@JimGriffOne]

The ball is trading rotational inertia for angle of attack as it hits a surface. It can trade it either way. So, you can actually make a spinning ball radically change its angle of attack, or a non-spinning ball trade angle of attack into rotational inertia (spin) when it hits a surface at an angle other than 90°.

[@cafebrasileiro]

I wanted to see you actually using the spin on your favor to get the ball to the other side. Sadly it wasn't shown... That would be satisfying haha

[@kvg4790]

Is it a requirement that it must be an elastic collision? Most of the instances of this happening (bouncy ball on hard surface or cue ball against billiard table rails) is one elastic body and one rigid body. Would pure friction between two rigid bodies allow for this to work? Also, how would two elastic bodies affect the angle? I’m guessing there’s some limit to slippage, so rotation won’t really be that much greater, but since both bodies deform slightly, the angle of reflection may be slightly different.

[@moosenensen1592]

If it wasn't an elastic collision the ball would simply stop moving after the collision as the energy would be transferred rather than reflected

[@kvg4790]

@moosenensen1592 That’s wrong because while nothing is perfectly inelastic, anything sufficiently not deformable is considered inelastic in a low force situation. Look up inelastic collisions. A great example is newton’s cradle.

[@neelpatel4934]

Great content. Loved the explanation. Just one suggestion. For better results with to visualize spin. You can consider painting the bouncy balls solid and put a bright axis across. May be an led too for dark shot?

[@TheVoidSinger]

While I understood the previous explanation, you're absolutely right that the discrete version makes it so much clearer what's happening. There may be some more meat in area (pun intended) of the discrete proof of Kepler's equal area law, but I can't quite put my finger on it yet

[@hans1120]

"I'm anthropomorphising the ball, but that's okay.": I love your extremely conscious and subtle way of showing awareness of metaphysical traps in your videos😁

[@AdmiralThumbs]

I'd be interested to see how this effect changes by reducing the cooeficient of friction between the ball and the sides. At first I thought of using a smoother/more slippery ball, but getting one that also has good bounce seems difficult, so perhaps an easier test could be simply soaping up the sides.

[@shadyarian]

Make both solid surfaces really hard, like metal, and use a hardened steel ball, or a glass ball. Both of those are quite bouncy and have low friction.

[@stickiedmin6508]

What about using the same basic ball, but change the surfaces it's bouncing on? Glass, sheet metal, wood, sand paper etc?

[@infra_Gray]

I love Steve's content because he never gives up at making an apparent contradiction into something intuitive. I am someone who genuinely believes in humanity and our genius, and so I also have to believe that we are all capable of intuitive understanding of the world around us. Understanding should really be easy, as long as the person teaching is smart enough to make it easy

[@1234jhanson]

Great video. I always encourage my students to subscribe to your channel. Brilliant

[@standupmaths]

Hexagonal‽

[@SteveMould]

That’s how I use the word, OK. Stop being such a prescriptivist.

[@ruzziasht349]

Banana

[@janmamu8721]

grapefruit

[@i_skin_infants]

Watermelon

[@Dosor72]

Interrobang‽

[@dylanlasky2389]

Is there any initial spin you can impart to the ball to get it to come out the opposite side of the cube?

[@martindinner3621]

Yes. With enough top spin it will generate lift and never actually hit the cube.

[@bogravel8551]

Pitcher's in American baseball know some of these physics as well to attempt to control the baseball for either the hitter to miss OR if hitter makes contact with spinning ball the 'bounce' (contact of) the bat is directional somewhat as ball travels back toward the field of play. I feel this plays a factor into how many 'out of bounds' the ball goes. Course the bat plays a spin factor into redirecting ball spin/direction too. Enjoyed video.

[@sergiorestrepo6657]

Thank you Steve

[@MichaelKakam]

7:58 - you say hexagonal, but show octagonal :) this really helps with understanding the cylinder though! thanks!

[@SteveMould]

Damn! I edited out the word hexagon about 100 times in this video. Can’t believe one slipped through.

[@cleon_teunissen]

@@SteveMould How did you search for occurrences? The reason I ask: the youtube machine-generated transcript is crazy good. So: with access to transcription software of that quality you can make it a text search, instead of you going through the audio. (Maybe Google's speech-to-text technology is available as part of Google Docs, I don't know.) Anyway, these days, if I want to search in a (youtube) video, I search the transcript.

[@CraftAero]

Every indoor (Box) Lacrosse player learned this one the hard way. It doesn't take too long to learn to count the bounces and predict the next bounce. Keeping your own balls clear because that ball is solid rubber and moving ~80mph. ie: if you chase the ball, you'll never catch it. If you know where the ball "will" be, you're a player.

[@gunterkewes9594]

Another inspiring video, well done👍. 1) I wonder if it's possible to "flip the grip", potentially with table tennis rubbers on the walls and a ping pong ball. Guess one would need in particular soft table tennis rubbers. 2) if yes, one could "generalize" things with respect to incoming spin experimentally quite easy, i.e. by using a table tennis robot.

[@sushi926]

I think the discrete explanation is easier to grasp but extrapolating to the infinity is always dangerous. I'm pretty sure that's what most (casual?) simulations do but I don't really feel convinced that rolling is just lots of infinitely small bounces. Really nice to see both explanations.

[@jorisev]

Still watching, but I'm wondering whether you could alter the friction between the ball and the surfaces. If you can reduce the friction, shouldn't the spin have no effect any more?

[@Vindolin]

Yep, what's with smoother ping pong or billiard balls.

[@aikumaDK]

Does Steve have access to a super slow mo camera? It would be interesting to see how the ball behaves in the microseconds where the ball's spin + friction makes it turn direction, especially at the two and four surface cases

[@hylje]

No, he only has access to particularly slow flying bouncy balls.

[@phidipidies]

I had some lsd and this was recommended when I came on KZbin. I’m so glad it did.

[@Sansei59]

The opening sequence reminds me of Michael Moschen, the American contact juggler who intuited much of what you're explaining, to use it in one of his performance pieces. Moschen is amazing: You can see him juggling in several KZbin videos. He was also the juggler who actually performed the contact juggling sequence that David Bowie did in the movie, Labyrinth.

[@weevilinabox]

Moschen turned bouncing balls into art. kzbin.info/www/bejne/p5uroJiapLiIjrssi=gkIQ3SP33gvpmorc

[@AGlimpseInside]

My three-year-old son has no problem doing this. 😂