I am here to sign a
document betting $10,000 that my last video is in fact, correct. This is the video in question. Some people may have missed it, but in this car, there is
no motor, no batteries, no energy source, besides the wind itself. And the counterintuitive claim, is that this car can maintain speeds faster than the wind that's pushing it. There is a physics professor at UCLA, who got in touch, to say that
he thought that I was wrong, that the explanation was wrong. You know, we went back
and forth a little bit, and eventually I said, "Well, how about we bet $10,000? "I can prove it to you. "This vehicle really can go
downwind faster than the wind." And to my surprise, he is
taking me up on the bet. - Look, no one is perfect, but you have a much lower error rate than most people on
YouTube, in YouTube Space. - Now, Professor Alex
Kusenko wanted this wager and all related discourse to be public. In fact, he suggested we get a celebrity to witness the signing. So I asked Neil deGrasse Tyson, Bill Nye, and Sean Carroll to be our witnesses. And they graciously agreed. - And Alex, I just wanna say, I agree with everything
you said about Veritasium. That generally I can watch it and not have to wonder,
is he gonna mess up? How am I gonna- - No, he's brilliant, no, he's brilliant. - I'm excited about this bet, because if I am wrong, then I wanna know. Like the whole point of the
channel is to get to the truth. And that is, I think why
we're all here today. And I think, you know, this is a great chance to sort of see. I'm going to summarize Alex's
main points in this video, but I'll put his full presentation here. - So let me first explain
what I see in the video. In the video, the vehicle
is operated in a gusty wind. Initially, you have the wind
speed exceeding the car speed, but then the wind speed is not constant. The wind speed drops, and
the car moves by inertia with deceleration for awhile. - So, basically Alex thinks a gust pushes the car to a high speed. And then when the wind dies, the car is going faster
than the wind momentarily, but it must be slowing down. - In fact, that will be my conclusion at the end of this presentation, that whenever you have
velocity faster than the wind, I'll actually show you in an equation, the acceleration is negative. The second effect is that
the wind in the video is measured at the height of about a meter or a meter and a half. But the propeller goes
to some three meters above the ground. - Now, due to interactions
with the ground, there is a wind gradient. Wind travels slower, close to the ground, and then faster, higher up. Now, Alex estimated that the
wind speed of the propeller might be 10 or 15% higher
than at the tell-tale. So it's possible that
the car could be moving slower than the wind at the propeller, and yet appear to be
moving faster than the wind at the height of the tell-tale. - Now, I think that
this is a small effect. However, in combination
with the previous effect, it just can make this more frequent, okay. And that completes- - Alex, if I remember the video correctly, Derek reports that they've achieved up to 2.8 times wind speed. That feels much higher
than what is possible here, unless the wind had picked up and then spontaneously sort of dropped. - Very, very good question. Okay, if you're going for the record, you probably will do many attempts. You will be sampling that gusty wind over, and over, and over, until you set the record, right? That's how you set the record. And on one of those occasions, you will get a nice strong gust, which is three times the air
that comes after it, okay? And that's when you will clock the record. And that's where that
2.8 factor will come in. - So what about the treadmill tests? These are conducted in still air. By moving the ground backwards, you're simulating a
perfectly steady tailwind. And if you hold the car
stationary on the treadmill, well, that's equivalent to the
car going exactly wind speed. Now, if the car can move
forward on the treadmill, that shows it can accelerate
faster than the wind. But Alex had multiple explanations, why these experiments don't actually show what they claim to show. - If you have this fluctuating
speed of the treadmill, and then if a human
just sort of steers it, it then can introduce unconsciously, a bias towards the desired result. - So, would it any be that
the guy with the spork is inducing the model craft to go across the lane now and then? And as you pointed out, I
would go downhill now and then. - Yeah, I'm sure, I'm
a hundred percent sure that the guy in the video
doesn't do it on purpose, okay. However, you know, if he
is expecting forward drift- - Yeah, oh he's trying. - Yeah, yeah absolutely,
that's exactly, okay, so there you go. - In the absence of convincing
experimental evidence, Alex turned to theoretical analyses, like one by MIT Aero
Professor, Mark Drela. But here too, he found problems. His main concern is that
the equation for net force includes the difference
between the speeds of the car and the wind in the denominator. Which seems to imply that when
traveling exactly wind speed, you should get infinite force. - Now, here's the real danger. Because if Derek drives
very close to the wind, that difference in speed goes to zero. If it's one millionth of one percent, that's like a nuclear
bomb exploding behind him. Then Derek is definitely
in trouble, right? So we need to find something
to save Derek's life here. This is serious, right? - But dividing by zero, come on, you guys. I never looked at that. - [Derek] Alex performed his own analysis, and found no such divide by zero problems. In fact, he found there's
no way for the car to accelerate at, or above wind speed. - The acceleration of
this craft is negative. So when we, you know, so it's possible to move the
craft faster than the wind, but it's not possible to
move it at zero acceleration which would be needed to
maintain constant speed. - [Derek] That is
basically where we left it. - That is right. - Okay, thank you, thank you, Neil. Thank you very much. - Thanks guys, thanks Neil. - So now it was up to me to
convince Professor Kusenko that Blackbird really can
go faster than the wind, When I posted about it on Twitter, Alice Zhang, who runs
Chinese Veritasium, said, "I think you lost Derek. I'm 80% on Alex's side now." What's amazing to me, is
that neither one of them had seen my attempts to replicate
the treadmill experiments. For the first video, I asked
my friend and YouTube maker, Xyla Foxlin, to make
a model downwind cart. - All right.
(wheels screeching) (laughing) - [Man] Oh, no! - Version one ended in failure, but Xyla was undeterred, coming back in a couple
of days with version two. - Is it feeling like it's gonna...? - Unlike these models, most
of her projects actually work. She is determined. So maybe this tells us something about whether you can actually go faster than
the wind, downwind. What was clear to me is that
I didn't do a good enough job in the first video,
explaining how Blackbird works and providing convincing evidence that it can really go faster than the wind in a sustained way. In my defense, I thought the concept was well enough established. Way back in 1969, Andrew Bauer build the first successful downwind cart. And he did it to settle a friendly wager with Aero Engineer, Apolo Smith. The bet was inspired by a
claim in a students paper from 20 years earlier. Now, Rick Cavallaro,
the builder of Blackbird was completely unaware of all this until after he built his cart. But other analysis have been published under names like the
push-me pull-you boat. So I didn't honestly
think anyone would doubt the vehicles operation,
much less bet me $10,000. But clearly, there is a need
for a deeper explanation. So I want to do that now
by responding to the points Alex raised. So first, let's deal with wind gradient. I mean, why didn't we measure the speed of the wind higher up? Well, the answer is because
it's already been done. They mounted tell-tales on fishing poles out to the sides of the
propeller and even above it. Now, although the lowest
tell-tale flips back first, all of the tell-tales do
eventually flip backwards showing that every part of the vehicle is going faster than the wind. Could this be because of a big wind gust that pushed the car up to high
speed and then the wind died? I don't think so. Even though I didn't have
a speedometer in the car for my runs, someone
on Twitter pointed out that we could use the
rotation of the back wheel to determine the speed
from the video footage. This shows that even after
the tell-tale flips backwards, the car keeps accelerating. Another thing I want to point out is that, if wind gradient or gusts were the reason that the car
travels faster than the wind, well, you'd expect the
tell-tale to jump around or at least not point straight back at me. But it consistently
does for over 30 seconds until I had to hit the brakes to avoid crashing into parked vehicles. But if that's not enough for you, when Blackbird achieved its record speed of 27.7 miles per hour in a
10 mile per hour tail wind, it was still accelerating. And we know this because there were multiple GPS units
in the car and wind speeds which were measured at the
height of the propeller at multiple locations. The highlighted section shows the ten second measurement period over which the record was set. Also in 2013, the U.S Physics
Olympiad Semifinal Exam asked questions about Blackbird. Like, can it go faster than
the wind downwind and upwind? The solution says, both
modes are possible. And with sufficiently low energy loss, any speed is possible. Now, I'll admit that the evidence I showed in the first
video was not definitive when gusts or gradients could have explained the observations. But now that you've seen this evidence, are you convinced that
Blackbird can go downwind faster than the wind without slowing down? Well, Professor Kusenko was not convinced. So I wanna explain how
the car works so clearly that no one, not even the professor, can doubt what's going on. The first thing to know, is that the propeller doesn't
work like most people think. It's not working like a windmill. It doesn't turn the way
the tailwind is pushing it. Instead, it turns in
the opposite direction, working like a fan to push air backwards. This fan is powered by the wheels, which are connected to the
propeller by a bike chain. So at wind speed, the
car can keep accelerating because the wheels turn the
fan, which blows air back, generating forward thrust. Now the big question is, to drive the fan, there must be a backwards
force on the wheels, which tends to slow them down. So why isn't this force bigger than the thrust from the propeller causing the car to slow down overall? Well, the answer is, the wheels are going so
much faster over the ground than the propeller is
moving through the air. So the thrust force
can actually be larger. I'm gonna do an analysis in the frame of reference of the car. And the important equation to know is, power equals force times velocity. So at the wheels, power
is input into the system by the ground moving underneath the car. The power generated is the force
of the ground on the wheels times the velocity of the car. At the propeller, work is done on the air, as the propeller pushes it backwards. The power out equals the
force of the prop on the air, times the speed of the car,
minus the speed of the wind. The prop is going slower through the air due to the tailwind. And if we assume no losses, then the power in at the wheels, equals the power out at the propeller. From this equation, we can see that the force at the propeller will be greater than
the force at the wheels. And since the propeller
is pushing air back, the air applies an
equal and opposite force forward on the prop. This is the thrust force,
which will be greater than the backwards force on the wheels. So, this car works like
a lever or a pulley by applying a small force to the wheels over a larger distance, the propeller can apply a larger force over a smaller distance. This is just like when you're
riding a bike going up hill, you move the pedals fast,
but with smaller force, to make the wheels move
slower over the ground, but with a bigger force. But now we've run into the
divide by zero problem, that Professor Kusenko warned us about. When the speed of the car is exactly equal to the speed of the wind, it seems like the propeller
can provide infinite force. That can't be right, can it? I mean, is our analysis flawed? The answer is no, for two reasons. First of all, this is
exactly what you'd expect theoretically, with any lever or pulley. If one arm of the lever is zero, then you can lift an infinite weight with any amount of
force on the other side. The catch is, it's
displacement will be zero. But second of all, in practice, there is a propeller efficiency term that is ill-defined when the propeller is not
moving through the air. - There's a better formula
for the prop proficiency, which is well-defined
in the zero speed limit. It makes an algebraic mess, but it's perfectly well-defined. - And then the divide by
zero problem is eliminated. But that equation makes the problem look more complicated than it actually is. You don't actually need aerodynamics. Here, I have a little
cart with a big wheel that rolls on two smaller spools. And what I'm gonna show is that when you have two media moving relative to one another, well then if this car is
in contact with both media, it can actually move faster
than their relative velocity. So as I push the board to the right, you can see that the
car goes down the board faster than the board is moving. If you look carefully, you'll see that the big wheel isn't turning the way that
the board is pushing it. It's actually rotating in
the opposite direction. That's just like the
propeller on Blackbird, which pushes back against the air, and that's how it's able to go faster than the wind downwind. Now you can build one of these
cars for yourself at home, or you can build a model downwind cart. I told you, Xyla was determined. Yeah, I'm gonna make the claim on camera. I like, I think it's gonna work this time. We're changing the propeller. - It has to work before we get kicked out of
the treadmill store. (laughing) (motors roaring) (chuckling) - [Derek] Does it work? - It totally works. - [Derek] Amazing. - Oh my god, it's so good. - Her fourth version of the
cart works spectacularly and it was designed to
be replicated by anyone using just a 3D printer and
a simple list of materials. She explains how to
build it with more detail on the engineering process on a video over on her channel. So go check it out. Now, professor Kusenko
has now conceded the bet, and he transferred $10,000 to me. So I wanna thank him for
being a man of honor, and changing his mind in light
of the evidence I presented, which is really not easy to do, especially in a public
debate like this one. Now I do not wanna keep the money. I wanna invest it in
science communication. So I'm holding a one-minute
video competition. I'll be awarding cash prizes
to the top three videos that explain a
counterintuitive STEM concept. I'll put some details
down in the description. What I love about science, is that disagreements are not problems. They are opportunities for
everyone to learn something. I learned a lot more about
Blackbird aerodynamics, and gear ratios than I knew before. I also learned that I should go into
more depth in my videos. I should make the evidence
overwhelmingly convincing and put in some equations toward the end for those who want that level of detail. I wanna thank everyone involved
in the making this video. Neil deGrasse Tyson, Bill
Nye, Sean Carroll, Mark Drela, Professor Kusenko and Xyla Foxlin, but especially Rick Cavallaro, the inventor and creator of Blackbird. He was a fountain of information, a constant source of support, and the man leading the charge
to help people understand this area of physics
for the past 15 years. Let's hope this video
puts the issue to rest once and for all. (electronics buzzing) The Blackbird craft all
started with a brain teaser. And this video sponsor, Brilliant, offers you a daily problem
to solve every day, like this one about gear ratios. If the first gear spins
10 times per second, at what rate does the final gear spin? Now I did this problem the hard way, but my wife figured out
how to do it the easy way. And that's what brain
teasers are great for. They get you thinking about the world, and they give you insights into problems you might think you already understand. Just think about how much more you would
understand in a year, if you got into the habit of solving one novel
unexpected problem per day. And while you're at it, why not take one of Brilliant's courses, like on computer science, neural networks, or classical physics. Even physics professors can benefit from some of the lessons
on frames of reference. Somehow I managed to go my whole degree without learning Lagrangian mechanics. So that's a course I'm
working through at the moment. It's a really elegant way
of solving physics problems. And I wish I had learned about it sooner. For viewers of this channel, Brilliant are offering 20%
off an annual subscription to the first 200 people to sign up. Just go to Brilliant.org/Veritasium. I will put that link
down in the description. So, I wanna thank Brilliant
for supporting Veritasium, and I wanna thank you for watching.
That little roller cart with the wooden bar has to be one of the best examples ever, explained the concept so well.
I’m confused then… if the propeller is actually acting as a fan powered by the wheels… where does it get its initial force from? The wind would actually make the propeller spin in the wrong direction it seems… do they just give it a push to get it going?
Oooh, now this is exciting.
EDIT: There's more drama behind the scenes here - Kusenko still thinks he's right and conceded because he believes he lost the bet on a "technicality".
EDIT2: Maybe that's not fair - his concession might actually be saying he could have claimed the win on a technicality, but he admits his initial position was fundamentally wrong? I dunno :D
This reminds me of the math professor who insisted that Ask Marylin was wrong when she said the player should always switch in the Monty Hall problem (for those who have no idea what I am talking about see: https://en.wikipedia.org/wiki/Monty_Hall_problem). Sometimes very smart people are overly confident and dig in their heels when they are wrong.
I'll believe this guy when he figures out Akinator the web genius.
I'm still confused about something. if the car gets past the speed of wind, then the wind stops, obviously the car would soon stop too. but if the car is going faster than the wind, how would the wind be pushing it?
Don’t sailboat to do this all the time
My physics knowledge is so far from adequate enough to have a dog in this fight but I sure am enjoying the back and forth debating!
It is such a counter-intuitive result! After some contemplation, I think I can explain it in terms understandable to someone (like me) with a undergraduate-level physics background:
From the frame of reference of the air, the ground is passing by below. Put a wind turbine on that ground, and it will pass through the air and extract energy from their relative motion, reducing the relative motion of the ground under the air in the process. Straightforward enough.
From the frame of reference of the air, this funny cart is doing the same thing as the wind turbine. It extracts energy from the relative motion between the air and ground, reducing the relative motion of air vs ground in the process, and uses that energy to propel itself. Without wind, there's no potential energy to be had, and the cart doesn't work. The part that's particularly counterintuitive is that this system works even when the cart's body is traveling at the same speed & direction as the air itself, but the principle is the same - so long as there's energy to be extracted from the air/ground motion, there's no reason the cart can't move in one direction through the air while the ground moves in the opposite direction.