Hacking The Kendama With Physics

The Rest Is Science

Michael Stevens introduces Hannah Fry to the kendama, a Japanese cup-and-ball skill toy, quickly transitioning into a deep dive into its underlying

Key takeaways

• A heavier, spinning kendama ball utilizes gyroscopic stability to resist torque, keeping the spike hole oriented downward during a catch attempt.
• Adam Savage machined a custom Delrin kendama with a centered string attachment to eliminate unwanted rotational torque on the ball.

Main topics

• Kendama physics and angular momentum
• Gyroscopic stability and precession

Notable quotes

"The trick with the cup is to just put the cup right underneath the ball."

Conclusion

The episode successfully bridges playful skill toys with rigorous phys

Transcript preview

Speaker 1 (0:00) Hello 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 Speaker 4 (0:08) favorite kind of Field Notes. There you go. Speaker 1 (0:11) Your very own kendama. Okay. Is it like Speaker 4 (0:15) a Speaker 1 (0:15) cup Speaker 4 (0:15) and ball? Speaker 1 (0:16) 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. 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. Right Speaker 4 (0:44) now in the package, it's sitting on that spike. So hold on a second. This is, it's like a more complicated version of cup and ball. Speaker 1 (0:53) 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. Speaker 4 (1:21) This is saying it's a classic Japanese skill toy. Speaker 1 (1:24) Exactly. And that's why it's called a kin-dama. So the kin is what you call the stick, the spike-cupped stick. And the dama is the Japanese word for the ball. Right. So it is a kin-dama. Those are its two parts. I Speaker 4 (1:37) 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. Speaker 1 (1:48) This episode is brought to you by Cancer Research UK. As Speaker 4 (1:51) if they weren't cool enough already, some astronauts have now added cancer scientists to their CV. They are making use of microgravity conditions in space to carry out experiments. That's because the same thing that Speaker 1 (2:03) lets Speaker 4 (2:03) astronauts experience Speaker 1 (2:04) weightlessness also affects how cancer cells grow, behave, and respond to drugs. And Speaker 4 (2:11) back down on Earth... Cancer Research UK is working with astronomers to explore tumours with technologies that are typically used to map the Milky Way, which I think... Gives you an idea of just how complex this disease is. Speaker 1 (2:24) That's right. There are over 200 different types of cancer. But by pushing boundaries and embracing the latest technologies, Cancer Research UK has helped double cancer survival in the UK over the past 50 years. Speaker 4 (2:36) And today it is continuing to save and improve lives around the world. For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org slash rest of science. Speaker 1 (2:56) I want you to try to spike the ball. Speaker 4 (3:00) Okay. Flip Speaker 1 (3:00) 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 Speaker 4 (3:29) mean, no, definitely not. Imagine if I'm amazing Speaker 1 (3:33) at this. That would ruin the entire episode because of what I'm going to show you next. Speaker 4 (3:39) Every time I try something new, there's a little bit of my brain which is like, this might be the Speaker 1 (3:43) thing. Speaker 4 (3:43) This might be Speaker 1 (3:43) what I was born to do. Speaker 4 (3:45) It's never the Speaker 1 (3:46) thing. It's never been true. Speaker 4 (3:47) It's never been true, but still, every time, every time. Okay. Actually, that's not true. Once I went clay pigeon shooting. Speaker 1 (3:52) And I really was. And you really were a natural. Amazing at it. And you continued to be amazing as you. Speaker 4 (3:57) Oh, yeah. I mean, I've only done it once, but I was Speaker 1 (4:00) 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. Speaker 4 (4:07) What is this? We're back to. I'm not getting it quite. It's not quite the right angle. Hang on. How many is the entire episode just watching me do this until I. Speaker 1 (4:18) Yeah, we cannot stop recording until you succeed. What Speaker 4 (4:21) I want you to do is I'd like to do this as many times as it takes to get it in. And then I want you to cut the episode so that it Speaker 1 (4:30) looks Speaker 4 (4:31) like I got it in first time. Okay. Speaker 1 (4:32) Yeah. All Speaker 4 (4:32) right. Speaker 4 (4:34) It Speaker 1 (4:35) might help if you were standing, to be honest, because it's hard to get the angle just right. Oh, that was close. Speaker 1 (4:44) You've made contact every time. Speaker 4 (4:46) This is fun to learn and difficult to master. Speaker 1 (4:50) Okay. Speaker 4 (4:52) I'm just going to do three more and then I'm going to give up. Okay. Damn it. Can you do it? Speaker 1 (4:57) Maybe. Speaker 4 (4:59) you Go on, go on, go on, go on, go on. God, you're Speaker 1 (5:03) so close. Speaker 1 (5:07) Okay. So. Was that fun to master? I haven't mastered it yet, but I'm having fun trying. It was definitely easy to learn. Yeah. Speaker 4 (5:15) Is that Speaker 1 (5:16) what they call it? Easy to learn? Fun to master. Fun to master. But let's try to master this faster. That was the challenge I gave to Adam Savage, one half of the Mythbusters. 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. Speaker 4 (5:41) 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 heavier. Before Speaker 1 (6:00) you use it, let me tell you what makes it... Do you think it's easy Speaker 4 (6:04) with Speaker 1 (6:05) 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 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 Speaker 4 (6:39) at the sort of apex of the ball, but inside it's not sitting in the center. Speaker 1 (6:44) Exactly. It's Speaker 4 (6:45) just like a kind of random knot. Speaker 1 (6:47) 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, cool. Get it spinning. Speaker 1 (7:12) And then we'll lift it. Would have been really cool if that worked the first Speaker 4 (7:18) time. Let's pretend this is the first time. Speaker 1 (7:19) Okay. Speaker 1 (7:24) Beautiful, right? Speaker 4 (7:25) I'm so jealous. Speaker 1 (7:27) It's beautiful. Okay, now you give it a try. I'll take out some of the spin on that string. Speaker 4 (7:34) 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, Speaker 1 (7:47) you just pull it straight up. Hang on, let me get it spinning. It doesn't need to spin too fast to be more stable. That actually was pretty close. I saw where the hole was. Speaker 1 (8:00) It's a bit unfair because what you'll notice if you look at the original Dama, it's got this gigantic, Speaker 4 (8:06) the Speaker 1 (8:06) holes gigantic and it's, it's even easier to catch. Whereas that hole is designed just to fit the spike dead on. Speaker 1 (8:16) Oh, Speaker 4 (8:16) man. We have to get me doing it because otherwise it doesn't make sense. Speaker 1 (8:19) Yeah, pull it up a little less high. Okay. Speaker 1 (8:24) 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 Speaker 4 (8:33) sorry, guys. I've got to do it. Speaker 1 (8:35) No, no, no. This is fun. Speaker 1 (8:38) If you were standing, I think it would be a lot easier because the trick is to pull it straight up. Speaker 1 (8:48) That I didn't think 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 journey that we took. Speaker 4 (9:21) And Speaker 1 (9:21) how Adam Savage made it amazing. That whole thing,