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Hello and welcome to SciShow Talk Show where we talk to interesting people about interesting things! This week we discuss why we know why we don't know how bicycles stay up with Minute Physics host Henry Reich. Special guest Jessi Knudsen CastaƱeda.

Hosted by: Hank Green
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 (00:00) to (02:00)

(SciShow Intro plays)

Hank: Hello and welcome to SciShow Talk Show, that day on SciShow where we talk to interesting people about interesting things. Today, we have on the show Henry Reich, the professor of Minute Physics.

Henry: I am not a professor, but I do do Minute Physics, and Minute Earth. We have new t-shirts.

Hank: Thanks for coming on the show. 

Henry: Thanks for having me. 

Hank: Yeah, so, so Minute Physics and Minute Earth are two of my very favorite YouTube channels in the world.

Henry: Thank you.

Hank: You make really great content and I don't know how you do it. You--you--you put so much interesting information in such small package. 

Henry: We just--we like, put it in a vacuum--

Hank: Okay.

Henry: --and like, suck out of all of the air from it and it just becomes vacuum sealed.

Hank: a word vacuum.

Henry: Exactly, that's what happens. So shall I show you some things today?

Hank: Yeah, well- well first, uh, you've been on the show before.

Henry: Yes.

Hank: But tell us more- a little bit about yourself.

Henry: About myself?

Hank: So, here's the thing-

Henry: Do you mean like about myself-

Hank: yeah.

Henry: or about what I do with science YouTube stuff?

Hank: So here's what I know about Henry, uhm, he said to me one time, "I wish that I weren't pretty good at a few things and that I was very good at one thing." And by pretty good he means that he is basically a champion marathon runner-

Henry: This is false.

Hank: an amazing player of the mandolin-

Henry: This is also false.

Hank: and a, uh- you are in the top one percent of runners, mandolin players, and physics-

Henry: Science YouTubey people?

Hank: Science YouTube people, yes. So uh, you are a trained physicist.

Henry: I am trained as a physicist. uh, I also train to run-

Hank: Yes.

Henry: in a different way, uhm and yeah I do play a lot of music and those are things that I enjoy doing. I wish I didn't have to sleep because then I would have more time to do all these different things. It's hard. I kind of will go and, in phases where like, this is the time where I'm doing lots of one of these- like I'll run a lot and then I'll cross country ski a lot and then I'll play mandolin a lot.

Hank: I forgot- I forgot about the cross country skiing.

Henry: Those things are essentially the same. Cross country ski is just running when you can't run cuz there's snow everywhere.

 (02:00) to (04:00)

Hank: (laughs)

Henry: It's basically the same thing.

Hank: Alright.

Henry: So yeah, I go in phases. And actually, I'm glad you brought up sports, because that's actually what I want to talk about today.

Hank: Oh, you're gonna talk about sports.

Henry: Some sports things.

Hank: Here on SciShow Talk Show. A show about science!

Henry: Well, there's gonna be some science in this sports conversation, don't worry. Don't worry, people! There is science.

Hank: (laughs) There's physics in everything, Henry.

Henry: So I need you to put this on.

Hank: Oh, OK, I'm gonna put on a helmet, which is worrying. Is this gonna be more or less dangerous than holding a scorpion?

Henry: I think it will be less dangerous than holding a scorpion.

Hank: Okay.

Henry: Although you were putting your face really close. To the scorpion's ... (head?).

Hank: (laughs) My head is bigger than yours. OW! It got my skin!

Henry: Okay, are you ready?

Hank: Ohh, that's a tiny bike.

Henry: It is a tiny bike.

Hank: That is a tiny, tiny bike!

Henry: It is a tiny, little bike. I wanna talk about-

Both: how bikes work,

Henry: why they stay up, why they don't fall over.

Hank: Yeah, cuz I, as far as I can tell, uh, no-- like, I... don't feel like anyone knows the answer to this question, in com--like, in completion.

Henry: That is, in some ways, a really accurate statement, but I think it doesn't mean what you think it means. So, we'll get- we'll get to it.

Hank: (claps) Henry.

Henry: uhm... so most people tend to think that bikes- uh if they know, like a little bit of physics they tend to think that bikes stay up because the wheels are spinning and because that means that-

Hank: Right.

Henry: -you have conservation of angular momentum and-

Hank: Mhmm

Henry: -and it makes it harder to turn. So most people think that's why bikes stay up.

Hank: Mhmm

Henry: -and there's demos where you have like a big wheel and you spin it up and you hold and it is hard to turn, but the force is just- the torque that keeps- it's just not enough, it's not a- and if that were the case also then, the faster you went the more stable the bike would be, which also isn't necessarily the case because you can ride a bike fairly slowly.

And the other thing actually because really what I'm talking about isn't even riding bikes, it's pushing bikes with no person on them. Cuz if you take a bike- I can't do it with this bike in the amount of space we have here but if you take a bike and you push it-

Hank: Mhmm.

Henry: -it can stay up on its own-

Hank: Yeah.

Henry: -like it'll kind of turn one way and it kind of self-corrects and doesn't fall over.

Hank: Some really good YouTube videos of motorcycles riding themselves. (laughs)

Henry: Yeah, exactly. Motorcycles- where somebody falls off of a motorcycle and it keeps going downhill

Hank: If it gets stuck on the throttle-

 (04:00) to (06:00)

Hank: If it gets stuck, I've seen them get stuck and they're just like "I'm coming for you! Whoever. Yes.

Henry: Yeah. So basically, the question is why do bikes- why are they stable, why don't they fall over? and there are essentially- people have done mathematical analysis- and what you mean by a bike is basically two wheels where you have a frame that has a hinge somewhere in it.

Hank: Mhmm. Right.

Henry: And there's a bunch of free parameters like how big are the wheels, where are they positioned, what's the mass distribution, where exactly is the hinge relative to the wheels, what's the angle, uhm, that sort of thing.

Hank: It seems to me that the placement of this hinge is a very important part-

Henry: It is.

Hank: -because it going backwards is not going to be-

Henry: It's harder.

Hank: -stable the same way.

Henry: Right. So, the thing with- the thing with bikes is that people figured out that the way that we build bikes, the way that our bikes generally work, up to a certain speed, you're going so slow that the bike just kind of spirals in and crashes. It doesn't correct it- like it corrects but not fast enough and it'll crash. Then there's a big speed range where as it turns, it'll self-correct and kind of wobble back and forth.

Hank: Right, this- like if you're pushing- cuz like the bike's gonna want to go forward-

Henry: Right.

Hank: -because it's going forward so if it starts to turn-

Henry: It'll go that way.

-but it wants to continue going straight.

Henry: That's actually not why it happens.

Hank: Does not happen.

Henry: No.

Hank: I was wrong! I was wrong.

Henry: It's not conservation of momentum in the forward direction either.

Hank: Well there's gotta be- that has to be some of it.

Henry: So the reason that we believe bikes that are designed the way our bikes tend to be designed work is because there's a feedback mechanism between turning and leaning. So what that means is that, uhm, because- because there's this angle on the fork, the bike isn't actually touching the ground, like, at the- at the end of if you extend the fork is

Hank: Right.

Henry: -it actually touches the ground slightly behind that and what that means is that if you lean the bike the handlebar turns. and what that means is that if the bike is going forward, it starts leaning a little bit, which would make it fall over, the handlebar turns and it then, basically the wheel comes back underneath it. So the wheel self-corrects and comes under the bike, bringing it upright again. So you'd t- you lean, turn the wheel, and the bike then self-corrects and then it might over-correct and go the other way but basically it's a self-correcting mechanism.

 (06:00) to (08:00)

Henry: And there can be gyroscopic effects from that, where when you turn it, it helps the handlebars turn, but people have built bikes where the wheels are really small or where they have a wheel that's spinning the opposite direction to cancel out the gyroscopic effect so the gyroscopic effect is-- absolutely has nothing to do with how bikes stay up. It can help them stay up, but it really doesn't-- it isn't the main thing--

Hank: Can we explain the gyroscopic effect real quick because... What is that?

Henry: Sure, the gyroscopic effect is basically conservation of angular momentum. Basically the point is like if I push-- if it's spinning and I push it up this way, it will actually turn that way so it, it gets done nine-- basically the torque takes effect 90 degrees out of phase, uhm

Hank: and this is how they actually move spacecraft in the air without launching propellants out of them. Not--

Henry: It is one way of doing it, yeah.

Hank: Not to push them but to change their orientation in space.

Henry: To turn, exactly. Yeah, you can use gyroscopes to do that. But the thing with bikes is that they're actual-- there are so many kind of free parameters with how you can build a bike, if your definition of bike is like "two wheels, frame, and a hinge" uh, that you-- people have designed bikes that have backwards forks that are stable, that will ride on their own. They just have to change like mass distributions and other things. People have designed bikes that have the hinge in the back that basically can ride backwards and be stable.

And this is what I meant when I say like "we don't know why bikes stay up" is that we don't know what like, actual collection of parameters for this-- what we mean by a bicycle that is necessary for it to stay upright. Like we know that if we change certain things about our design, it'll start to fail, but if you change them far enough it will start to succeed again.

Hank: (laughs) What I wanna know is did you walk into a bike shop and say "I need a tiny bike."

Henry: Yes, I did.

Hank: (laughs)

Henry: In fact the bike shop is like, right underneath our office.

Hank: Yeah

Henry: I walked in there and--

Hank: Can I just borrow a tiny bike?

Henry: They actually rent out-- this is a kiddie bike, I think that it's for kids that are too little to like--

Hank: yeah they kick, they push

Henry: --to keep pedalling and stuff so they kick with their feet. It's perfect.

 (08:00) to (10:00)

Henry: It's perfect--

Hank: It's perfect for a demo

Henry: --for demos about bicycles

Hank: You don't need pedals for it to be a bicycle.

Henry: That is very true.

Hank: Not on the list of things you listed.

Henry: It's not on the list of things. Now you know why we don't know why bicycles stay up.

Hank: (laughs) Thank you.

Henry: Yes. Do you want to try and ride it?

Hank: I don't. Cuz you rented it and I don't wanna injure it.

Henry: Well I think-- I think the rental is normally intended for-- for being used to be ridden.

Hank: But not by a fully grown man.

Henry: That-- that's true. Yes. Smaller human beings.

Hank: Do you want to meet an animal?

Henry: Yes.

Hank: OK.

Henry: Let's meet an animal.

Jessi: So this is Rook the raven.

Hank: Hi Rook the raven.

Henry: Hello.

Jessi: There's someone behind you too.

Hank: It's so easy to have no idea how huge you actually are--

Henry: So big.

Jessi: They're really big.

Hank: --until you're right up...

Jessi: We have Rook because he was found in the wild with a broken wing, and that wing was already healed, it was calcified, we couldn't-- we couldn't re-break it, we couldn't do anything to it. It has to just stay there. He can't extend his wing, he gets thrown off balance a lot.

Henry: He's missing his wing tips feathers as well.

Jessi: I actually trim those off because it's held in such an awkward way that tho-- the primary feathers get stuck between his legs and he ca-- he trips on them. So I actually trim those off to help him out, uhm, a little bit more.

And, the stuff that I have going on down here, those are anklets, jesses, and a leash, because he can't fly and if he does get spooked and jumps off my hand he'll crash land. He'll land on his face.

Henry: So, as bad as that looks it's actually safer to have him land--

Jessi: Exactly.

Henry: --or not land, be hanging--

Jessi: To hang and then get righted back up. Exactly. Yeah.

Hank: I'm sorry. Hi.

Jessi: Yeah, there you go, buddy.

Hank: "I wanna be free again."

Jessi: "I'm angry."

Hank: Can't happen.

Jessi: Yeah, can't happen, he was extremely malnourished when they found him and picked him up. They brought him to a rehab facility and they fed him up, I mean it took-- it took a couple months to get him healthy again, to the point where they knew that he was gonna make it and live, but once they realized that "OK, he's gonna live, now where's he going to live?"

Hank: Mhmm.

Jessi: So they called us up and we said "yeah, we'll-- we'll work on taking him in" uhm, that's a pretty big uhm, responsibility to take on, an adult raven from the wild.

 (10:00) to (12:00)

Jessi: They are incredibly intelligent, I mean they're up there with great apes and dolphins and humans for intelligence. These guys do things like they point, they use their beak to point at objects, and they use communication where they'll pick it up and show it off to another raven, usually their mate saying "hey look what I found look at this thing, look what I found right here."

Hank: Yeah. Good try.

Henry: So... so does he get depressed then? From being... you know, not in the wild?

Jessi: From not being in the wild? That's definitely a concern. So, we give him a lot of behavioral enrichment, so things that will keep him really busy-- (to Rook) Are you trying to figure that out?

Hank: Yeah, he's like "this thing is attached to my feet."

Jessi: Yeah, well when I take the leash off, he can actually pull those jesses out of there. So he's used to like "nope, and that goes on the floor." (they laugh) But he can't do that when we have the leash on. uhm. (to Rook) I know. Are we done? We might have to be done.

Hank: OK, Rook had to go.

Jessi: He had to go, he was not enjoying this. I mean, that's a lot to ask from a wild raven, to be surrounded by people with lights on you and uh, to have the jesses and the anklets on it's just making him more angry, so that's understandable for him.

Uh, the reason that we continue to work with him and don't just leave him alone is to make sure that he doesn't get depressed. Uhm, we want to give him a lot of behavioral enrichment so-- that means making him work for his food, which is what he'd be doing in the wild. He'd be looking for a mate, or he would be-- found a mate and he would be monogamous, these guys mate for life, or for very very long times, and then they form little social hierarchies within the group.

We would like to supply that for him. It's difficult in captivity, it's like an arranged marriage, you never really know what's gonna happen, but um-- so social interactions and then finding food, that's gonna be huge. So we try and hide his food all over the place in his enclosure, and constantly try and stimulate his mind. Uhm, but we do have an enclosure that's like, it's like a handicapped enclosure. You know, I'm actually happy with his progress, we've had him for about a year. And so I'm pretty-- I'm pretty happy with where he is now.

 (12:00) to (14:00)

Hank: No idea what happened to Rook?

Jessi: We don't know what happened to Rook. Nope. It's up in here. It just-- You know he could've been hit by a car, he could've been bit by a predator, he could've fallen out of a tree the wrong way, uhm, he's-- he's pretty good at maneuvering around, but he's not-- he hasn't really perfected-- I don't know if he can perfect it. I-- I wonder how old he is, we really have no way of telling, he's an adult. And, so, was he out in the wild for three months? Or was it, you know three years? We don't know how we-- He was-- he was really malnourished, but who knows how well he could've gone around and eaten you know, berries or whatnot and--

Hank: Yeah, it's amazing how long and capably animals can sometimes survive after being... you know, very very sever-- seriously injured. Emilie Graslie once showed the the bone of some kind of ungulate that had had a compound fracture that had healed around the fracture so that like-- and the animal continued to live for years--

Jessi: Wow.

Hank: --with the bone sticking out of its leg.

Jessi: Wow. Animals are pretty darn resilient.

Hank: and-- and just very-- like I am... a weakling. I'm just like "ah, my toe hurts..." (more whining)

Jessi: It's alright, it'll heal and calcify around and you'll survive. Just wait.

Henry: If it got to the point where like you were going to be dying though You're inspiration to do more would definitely kick in. You would--

Hank: Right. Maybe. but I do whine an awful lot as it is. I should take-- I should take a hint from Rook. "I don't care. I'll try to fly. I haven't been able to fly for years but I'll continue to try."

Jessi: I'm gonna try.

Henry: I have a question. Is there any potential to do like, do prosthetic wings in situations like this where you can help or is this a case where it was just so badly like, muscle damaged that there was no hope for--

Jessi: It was-- There was no hope. There was no hope going back. It was broken in several places and just so extensively calcified, like it had created this big-- just it cannot extend so there's really no changing that.

 (14:00) to (15:34)

Jessi: I mean the best that we could do for it, I mean either leave it there and clip it off but if there was any further damage: amputate the wing. I mean that's where you would go with that. uhm, there is this thing called imping though, that if you were trying to rehab a bird to go back out in the wild, if they have been um, I don't know hit by a car or stripped of their primaries for some reason and they're just not growing back fast enough, you can imp them by taking other feathers from another bird that didn't make it, and you glue them on-- and so they-- so their bone is fine but if you glue them on there, they can use those until their other ones grow in and you can actually release them like that and the other primaries will grow in and push those fake ones out. So that's really awesome.

Hank: Huh. Weird. Cool.

Jessi: Yeah. Prosthetic would be really neat but I don't know-- You have to--

Henry: That's basically what that is though.

Hank: Yeah, prosthetic feathers.

Jessi: Yeah, you know, but no bones.

Hank: Feather transplant, yeah.

Hank and Henry: Feather transplant. (Hank laughs)

Hank: Great well thanks for sharing Rook with us.

Jessi: Yeah Yeah.

Hank: If you want to see more of what Jessi's up to, you can check her out at and Henry is at Minute Earth and Minute Physics. I'm Hank, thank you for watching this episode of SciShow Talk Show. If you want to keep getting smarter with us, you can go to and subscribe.