Hacking The Kendama With Physics

0:01Hello and welcome to The Rest is Science. I'm Michael Stevens. And I'm Hannah Fry. And today on Field Notes, I have brought you a gift, Hannah. My favorite kind of Field Notes. There you go. Your very own kendama. Okay. Is it like a cup and ball? It's a cup and ball. It's a style of the cup and ball game. You've basically got this wooden stick with two cups on either side and one on the bottom and a string attaches. I got scissors for you. Thank you. You've also got a string that attaches a wooden ball to the stick. And you just get to swoop the ball up and catch it in the cups on the stick. Or because there's a hole in the ball, you can catch it on a spike.

0:44Right now in the package, it's sitting on that spike. So hold on a second. It's like a more complicated version of cup and ball. Exactly. Where you have more options. You've got more options. You can do a lot more tricks. And you can also do a lot of physics with it. Are you really good at this? I don't think so. Okay. Those of you in this room that saw me practicing while Hannah was out of the room, you know how bad I am. I did try to get really good. When I was on tour with Adam Savage, a fan gave me one and I just played with it all day on tour. And I thought I got pretty good. This is saying it's a classic Japanese skill toy.

1:24Exactly. And that's why it's called a kendama. So the kin is what you call the stick, the spike cupped stick. And the dama is the Japanese word for the ball. So it is a kin dama. Those are its two parts. I mean, thus far, it's been quite a struggle to even get it out of its packaging. So I'm not sure this is going to go that well.

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2:56I want you to try to spike the ball. Okay. Flip it up and I'll give you one tip. It helps to start by just letting the ball hang all the way down to the ground and then unwrap it a little bit more. There we go. And then pull it straight up because the hole is on the bottom of the ball. So if you pull it straight up and you get it to fall onto the spike, it might... Okay, yeah. So you've got the technique down, but that hole was not in the right place. I mean, no, definitely not. Imagine if I'm amazing at this. That would ruin the entire episode because of what I'm going to show you next. Every time I try something new, there's a little bit of my brain, which is like, this might be the thing.

3:43This might be what I was born to do. It's never the thing. It's never been true. It's never been true. But still, every time, every time. Okay. Actually, that's not true. Once I went clay pigeon shooting and I really was. And you really were a natural. Amazing, Addo. And you continued to be amazing as you. Oh, yeah. Okay. I mean, I've only done it once, but I was amazing. All right. Let's try this one more time, at least. Okay. So you're hitting the ball with the spike. All right. You want to get into the little hole. All right. What is this? We're back to it. I'm not getting it quite. It's not quite the right angle.

4:14Hang on. How many? Is the entire episode just watching me do this until I? yeah we cannot stop recording until you okay let me do what i want you to do is um uh i'd like to uh do this as many times as it takes to get it in and then i want you to cut the episode so that it looks like i got it in first time okay yeah you're right it might help if you were standing to be honest because it's hard to get the angle just right oh that was close you've made contact every time this is this is fun to learn and and difficult to master yeah okay okay okay i'm just gonna do three more and then then i'm gonna give up okay damn it can you do it maybe go on go on go on go on god you're so close

5:07okay so was that fun to master uh i haven't mastered it yet but i'm having fun trying it was definitely easy to learn yeah is that what they call it easy to learn fun to master fun to master but let's let's try to master master this faster that was the challenge i gave to adam savage one half of the myth busters i said can you make me one that makes it easier and so he built me this this is a machined delrin kendama you'll notice it's a lot heavier okay here's the thing you just throw that into conversation oh yeah adam savage made me a version for people who are watching slash 18 for this who haven't come across adam savage he's like a massive massive hero of mine gigantic hero of mine okay let's give this a go all right it's much heavier the ball is heavier and the handle is before you use it let me tell you what makes it special do you think it's easier with this one is it going to be easier it should be easier for a couple of reasons okay so first of all it's made of delrin which is a plastic that you can machine on a lathe and we made this to uh hopefully use physics to make it easier so for example the string is attached to the very center of the dama of the ball so when you lift it you don't put a torque on the ball so it comes straight up with that hole facing the ground and it doesn't get twisted as much because in here it might be at the the sort of apex of the ball but inside it's not sitting in the center exactly it's just like a kind of random knot exactly so you kind of have to like learn the personality of the kendama whereas with this one the physics should help us secondly the dama is really heavy and it's built to spin so you can use gyroscopic stability to keep that hole facing down let me try this one okay go get it spinning and then we'll lift it would have been really cool let's pretend this is the first okay

7:24beautiful right i'm so jealous it's beautiful okay now you you give it a try i'll take out some of the spin on that string okay okay so let me just think about this then so because you've got the gyroscopic effect of it because it's spinning around it means that the ball is less likely to rotate in any axis other than the one that you can spike yeah you just pull it straight up hang on let me get it spinning it doesn't need to spin too fast to to be more stable that actually was pretty close i saw where the hole was it's a bit unfair because you'll what you'll notice if you look at the original dama it's got this gigantic the hole's gigantic and it's it's even easier to catch whereas that hole is designed just to fit the spike dead on oh man we have to get me doing it because otherwise it doesn't make sense yeah pull it up a little less high okay it helps also to imagine that you're stabbing the ball you're not hoping that it lands on the spike you're hoping to shove the spike into where the hole is i'm sorry guys I've got to do it.

8:35No, no, no. This is fun. If you were standing, I think it would be a lot easier because the trick is to pull it straight up.

8:48That, I didn't think... How many put? That was amazing. That Dama went up like so high above the spike and yet it still fell right on the spike. Because again, it's so heavy. And when it's spinning, it's just so much less likely for the hole that you need to go out of the way it's like here this is how i was and this is how i still am and it goes right on the spike so i treasure this for the physics behind it and for the whole the whole journey that we took you can watch adam savage made it for me thing that that whole thing exactly um you can watch an entire hour-long video on his tested channel where I tell him, I just would really like the Dharma to be heavier.

9:34And he's like, okay, I've got a special machine that can build spheres on a lathe, whatever radius you want. And the same tool could be inverted to make the cups. It can do it reverse sphere as well. And then the, uh, the knurling on the handle is actually done with a lathe attachment for making the threads on a screw. So wait, what's what material is it? It's like a plastic yeah it's a plastic called delrin okay and it's a it's a dense plastic that you can machine like if you put a softer plastic on a lathe the blade would just grip it and the whole thing would break it'd be very dangerous but this you can just put a little blade on it while it spins and you just cut pieces away you cut strings of it away it's amazing but then you end up with something that's more robust than if you 3d printed it for example more more robust yeah exactly exactly it's very robust and it will um it's it has some give it won't wear away even it'll even last longer than wood i mean no surprise it's plastic but if i come back in 10 000 years i'll still be there it'll still be here that's right still be relatively there will be you know whatever humans evolve into what 10 000 years you said yeah and it won't be that different but they'll still be struggling with it and what about these bits can you do like cool tricks with this stuff yeah try to catch the ball in a cup okay like trick with the cup is to just put the cup right underneath the ball oh it turned it turned there's not enough friction do you know what this really reminds me of the cup and ball thing so um when it comes to robots uh you'll have seen loads of really impressive videos of robots doing like backflips and then running through forests and all of this amazing stuff.

11:24The thing about those robots is that they're pre-programmed. It's like a script that they're following. So they can adjust it slightly for wind and terrain and things, but they're essentially like someone has sat down and worked out exactly how they should move. If you get a robot to feel its way through a problem instead, they struggle way more. Oh, like a human. like a human and a few years ago i got to go in i mean i've been several times but in particular a few years ago i went to go into the robotic labs at deep mind where they were at the phase where they were teaching robots how to play cup and ball oh that was what they were using as a way for the robot to understand its own limbs to understand the environment to understand physics gravity and so on and so on like the solidity of different things and so it's sort of in a little In a way, it was a little bit sad, but this robot was just there.

12:20I mean, it was just an arm, not a whole body. But this robot was just there over and over again like this. Fail. Okay. And then reset. And then over and over again. And then fail. And then reset. And it would be there overnight, day after day after day after day after day after day. Just like, and go. You want to help it. And fail. Even though giving people advice on this thing probably just annoys them. Let's see if I can catch it in the cut below. oh so close so close i feel like some parts of the savage designed kendama are harder for example we already talked about how the hole is so much more narrow i think the cups are also less deep oh um this is great though this is uh this is my kids don't need an ipad anymore they've got one of these yeah exactly you know what toy my daughter recently fell in love with Tell me paper yo-yos, just the stick that's got a lot of paper, a long ribbon of paper wound up all around it.

13:23And so you can throw it like this, right? And it shoots out. And she just played with that all day. And then at her birthday, she wanted to give one to every kid that came. And I'm like, I love that this paper on a stick is still so exciting to kids who have literally the entire world and history of media at their fingertips. they're still like, the properties of wrapped paper is so cool. Right, but that's who we are. We love experimenting with our environment and finding things that surprise and delight us. Exactly. It's all about, I mean, basically back to the robotics thing, the way that our brains work is that we make predictions about what's going to happen and then are delighted when the world meets our predictions exactly what the robots are doing um yeah and children and robots alike doing the same thing and adults so you can take that home and you can practice oh i'm gonna i'm gonna and you'll be amazed we we actually made a custom kendama with kendama usa for the curiosity box years ago i could not find one in the time that i was packing but we put um holes in all the cups so that you could take these additional spikes and make it make it a spike everywhere also doubles as a way to beat people up.

14:38Exactly. It became a weapon. If they were trying to bully you in the playground. Yeah, it became a weapon. Are they like Kandama world champions? Oh yeah. You should look up what some of the pros do. They are spinning the string around their finger and it's called juggling. I don't have anything else to say about it except that I really treasure this as a memory of a build with Adam Savage, a deep dive into the physics of how it works, but also the lesson that even when you try to hack the physics and make the easiest condomita catch on a spike you're still not that good at it it still takes that human flesh neural network a lot of tries to get the knack of it to get the physical memory of it hey look my main memory for this is that i managed to do it while standing up you did i did yeah i did manage on your very first try because of the way the editors put the clips together thank you guys this here's a problem this one it's changed i feel like the white plastic has expanded or the black has contracted because look if i try to catch it on the cup it rotates off like it doesn't have enough friction anymore at the end of the day things that are spinning are more stable because they they accelerate differently in response to forces.

16:01Imagine you are, you're a piece of mass that's going around in a circle. That's parallel to the plane of the floor. Okay. Let's say when you come around here to the front, I put a force on you right from above. Like I'm going to push you down to the ground. What happens? Are you going to go like this? Whoa. And just suddenly start spinning now perpendicular to the floor. No, Now, your velocities combine. And now you go, you keep going counterclockwise around, parallel to the floor, but you're also going to be going down a little bit. You're going to go a combination. You're going to go diagonally.

16:37But look at this diagonal motion. Now you go down and then you come back up and back down. So if you were, if you imagine that you were an entire disc, what's happened is someone has put a force in the front and you've moved that way. so the effects of the force are translated 90 degrees ahead in the rotation okay so someone pushing right here feels an enormous amount of resistance because normally when we push things they move in the direction of our force but not a spinning object it goes like that right so any little torque any any rotating force applied to a spinning object is going to cause it to behave differently, it's going to cause a lot of resistance.

17:19So it stays stable where it is, unless you put a lot of force on it. But if it's just in a gimbal or something, then as the ship moves, the gyroscope goes, nah, nah, move around me. I've got way more inertia, or it feels like I have way more than you do. So that's why they're so stable. Which is why you have them on ships, on ships to make a little gyro compass and it just keeps pointing north because as the ship moves, the little like axles that hold this gyroscope in place, they're like, come with us. And it's like, you got to try harder than that. And they're like, okay, we'll just go around you.

18:00And it just keeps pointing north. Right. But also I guess on a ship in particular, you've got these, so it's not just that you've got the one force pushing there at one point because you're, you know, a moment in the future, you'll have another force in that direction. And if you are feeling that force, but averaging it with the existing velocity of it turning around in a circle, then you've just got a little bit more time, I guess, right? Yeah. And you have to make sure that the object is spinning fast enough that it barely gets moved by those forces so that it's spinning so fast and it's so much or it's so heavy that a little touch here causes the smallest change 90 degrees ahead gravity tries to to make it fall over this way and it goes like that and gravity goes okay fine i'll i'll tip you over that way and this way and then it's processing and that's why we see gyroscopes process like this where they point their tops around in a circle until eventually they run out of energy should you go to a break yeah let's go to a break see if you can guess what i'm going to be doing in the break and we'll answer your questions when we come back certainly will we are getting a statement from number 10 on that flight with 350 passengers on board the great game strategic struggle between Britain and Russia We assess, uh, Prime Minister.

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21:06Welcome back. It's time for your questions. Certainly is. First of all, I got one from Ben from email. Ben asks, are there any mathematical advantages that can be used when swimming? Is it possible to swim faster with the power of math? Okay, look, Ben, you're on my own turf here. We're in fluid dynamics, right? And math. I'm happy, happy. I'm happy, happy. Okay, here's the thing, right? When you are taught, did you do swimming lessons? Oh yeah. Oh yeah. Okay. Show me how you're taught to hold your hand in swimming lessons. Like a little cup? Math says no. Math says no. What should I be doing?

21:46Okay. So here's the thing, right? If you, if you had like paddles for hands, right, you would, you would do way, way better at swimming. This is one of the reasons why you look at sort of professional athletes, professional swimmers, and they do, they should have got these shovels, like giant shovel hands. Oh, right. There's still gaps between the fingers, but they keep them spread. They keep them spread because you can make it bigger. And the reason why you can make it bigger is because, I mean, if you go too far, then yeah, the water... It's like a comb. But closer... But closer, you can get little eddies that appear between your fingers so you can increase the size of your hand.

22:21Without more flesh. Without more flesh. Oh, wow. Right? So what you should do is basically like a natural hand position. Uh-huh. And there are real legitimate scientists that have looked into this, by the way, that have done all of the calculations about what your hand should be. So it's essentially your natural resting hand position, that is how you should be shoveling water. I bet this isn't true, but I've always wondered why our resting hand position is the way it is. Why when I flex no muscles is my hand not like this or like this? Why is it kind of this weird intermediate thing? and I haven't heard a definitive answer.

23:02There's just a lot of speculation that like, well, that gives you the best leverage on both sides. The muscles only have half as much to do from rest versus like if my hand was always at rest like this, then closing it would require a lot of really long muscle that had a lot of work to do against the natural phase and vice versa. So it's like you're in your average position, essentially. But the fact that our resting position is ideal for swimming and pushing fluids makes me revisit the aquatic ape theory, huh? We evolved to be water people. I love the aquatic ape theory. Yeah. It's one of my favorite examples of how you can tie together evidence that basically makes it look like you can prove any argument by induction, essentially.

23:52You could say, look, we've got blubber like attached to our skin. Do you know who else has got that? Whales and dolphins. We breathe air. You know who else does that? Whales and dolphins. We don't have any hair on our bodies. You know who else doesn't? Whales and dolphins and so on and so on. Exactly. And then as long as you give it a really clever name, aquatic ape, stoned ape, like people go, oh, wow. Now it's got like a banner. It's got a team name. I get behind that. Yeah, unfortunately don't get behind it because there's way more evidence that says that we weren't aquatic apes than that we were.

24:29Okay, other things you can do with swimming. Basically, a lot of it is about streamlining your body. So you want to be long and thin. The more streamlined you can be, the better. So there were swimsuits a few years ago that would compress your body as much as possible. You have to literally squeeze yourself into them because the smaller the surface area, the less drag that you'll be creating. There was also swimsuits where they had, okay, so sort of a flotation effectively in the back that would keep your legs up higher, again, to reduce drag. And that would have certain little inlets on the surface of the costume that would allow the water to slip off it much easier.

25:11At that point, all of the records for swimming just went down, down, down, down, down. And then I think they banned them. I think they banned them and now we're sort of back to like, no, what's your flabby little flash body going to do with this track? Going to do on its own. Yeah. But that's the main one. The main one is, yeah, swim with a straight line. That's shortest distance. Mathematically, yeah. That's a good one. And swim with your hand like an aquatic ape. Okay, question for you. This is from Neil Fresh, who sent in an email, despite his name sounding very Reddit-like. Since Jupiter is a gas giant, what would happen if you flew right through the middle of it?

25:47You would not make it through. We've actually learned a lot about the inside of Jupiter recently. Before Juno went around Jupiter, it launched in 2011. I don't know when we got the results back, but before 2011 for sure, people did not know what was in the middle of Jupiter. We still don't really. But there were kind of two competing theories. One was that there's nothing. It really is a gas giant. It's just gas all the way down. The other theory was that, no, it might have a big, hard, rocky core. Like if Earth was 20 times more massive, it would have enough gravity to hold on to hydrogen. Maybe that's what Jupiter did.

26:29Maybe it has a surface. It's a rocky planet that's surrounded by, you know, hundreds and hundreds of miles of gas. So the Juno satellite orbited around Jupiter measuring its gravitational anomalies across the surface to kind of figure out what's underneath and what its sort of composition is and how symmetric it is. And as it turns out, it is not only it's not just gaseous all the way through. It doesn't have a core, though, either. The answer is somewhere in between. It's got what they call a fuzzy core. And it maybe isn't solid in the middle, but it's incredibly dense. We don't even really know what hydrogen in this sort of super critical phase is going to behave like.

27:15We call it a liquid metal, but it's not like mercury. It's probably not metal colored, but it's very electrically conductive. That's what makes it a metal. And so that might explain Jupiter's extremely strong magnetic field, that it's this very fast spinning, electrically conductive ball. So if you tried to just fall into Jupiter and shoot out the other end, you would hit this dense ocean, this slowly more and more dense gradient of hydrogen that became more and more dense. It doesn't have a surface with an atmosphere above it. It just becomes more and more thick and dense as you get to the middle.

27:56Hold on there. You said you wouldn't be able to pass through it. So, okay, I accept that it's like dense and thick, but has it ever become solid or is it still liquidy? It's still liquidy. We're not entirely sure, though. There could be the heaviest metals in the middle, but what state they will be in under the kind of pressure they would be in? It's not like, oh, yeah, eventually you drop a coin, whoops, down onto Jupiter. It's going to land on this hard surface in the middle. It's going to get lost in the fuzzy, gurgling mess that is the gradient of its density. So if you tried to go through it, there's no known material that would not get crushed up and phase changed in the middle.

28:44Right. Well, there you go. That's my ambition off the cards. It'd be a way to go. I was about to make a reference about swearing inside it, but that was a different thing. That was a different thing. It's a different thing. The first to say a bad word in Jupiter's gooey middle. Wait, the Juno probe, where did it actually go? Well, I know Jupiter for sure, but it didn't go to other places. Did it go in any of just past? It had this big elliptical orbit and it would come really close to Jupiter and then fly way back out. And it did that like scan thing where it scanned like slices over time so that we could learn from how its speed changed very slightly, like even by a tenth of a millimeter per second.

29:30We could tell based on the radio waves it sent back how it was being changed by what happened to be underneath it on Jupiter. So we could get this really detailed look at what the density of the mass is where. That's amazing. Yeah. And then presumably it flew off into space. It didn't come back. No, it didn't come back. But I don't know if it... It's not the one that went off to other planets later, was it? It does just feel like a bit of a missed opportunity to not nosedive into it at the end. I guess we've crashed them into Saturn. We don't want to bother any potential life forms that might live in places.

30:06So I know that we won't crash probes onto the jovial or Saturnine moons. Just in case. Just in case. But I don't know. Juno may have crashed into Jupiter. I don't actually know its final fate. Let me look it up. Did it get slingshot off eventually? Is it still orbiting? Where is it now? It's still alive. It's still orbiting Jupiter. It's still there. Oh, good for Juno. Good for Juno. It's approved to continue operating until its hardware finally gives out and contact is lost. Eventually it will go into Jupiter. Okay. All right. But it will be a slow natural death rather than a controlled dive.

30:41I wonder how long it could give us reports of what it was detecting as it fell into Jupiter. I'm sure there's not like a big 4K camera on it so we could really see the insides. I wonder how quickly it gets dark. Inside Jupiter. Inside Jupiter. On the sun facing side as you fall in, when does it become dark as night? But you're still falling. Because the clouds. Clouds. I mean, it would be discolored pretty quickly, wouldn't it? But I mean, how quickly? Because it's mainly hydrogen gas and other gases, but you could probably fall for 200 miles and still see out into space. I'm really scary about that.

31:24And be like, I'm still not even close to breaking the kind of outside. Amazing. Okay, last question. This is a question from Matt, who asked this question on Spotify. Matt says, which books would you recommend for an adult interested in getting back into maths, stats and statistics some years after high school? I would love to hear your answer to this. Okay, right. Here's the thing. I think textbooks are literally designed for this, you know, that's the whole point is to start you at the beginning and walk your hand through and pick up incrementally on the ideas one after the other. There are a few textbooks that I think are terrible.

32:02There's quite a lot of them I think are terrible. the famous okay the famous american one the calculus one hang on the stewart book for calculus right this literally brings me out in hives maybe it's just the way my brain works but it just oh god it's been so long defining limits before you actually get to any of the juicy delicious stuff okay in a really really laborious way it's like you know what if i learn how to play the piano as an adult, I don't want to spend four years on scales before I get to just try something. Maybe I'm not going to be very good at it. Maybe I'll only be able to do like a little section of it, but I want to like play with it.