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00:00I am here to sign a document betting $10,000 that my last video is in fact correct.
00:08This is the video in question.
00:10Some people may have missed it, but in this car there is no motor, no batteries, no energy
00:15source besides the wind itself, and the counterintuitive claim is that this car can maintain speeds
00:22faster than the wind that's pushing it.
00:25There is a physics professor at UCLA who got in touch to say that he thought that I was
00:31wrong, that the explanation was wrong, you know, we went back and forth a little bit and
00:35eventually I said, well how about we bet $10,000, I can prove it to you, this vehicle really
00:43can't go downwind faster than the wind.
00:45And to my surprise, he is taking me up on the bet.
00:48Look, no one is perfect, but you have a much lower error rate than most people in
00:55YouTube, in YouTube space.
00:56Now Professor Alex Kosenko wanted this wager and all related discourse to be public.
01:01In fact, he suggested we get a celebrity to witness the signing.
01:04So I asked Neil deGrasse Tyson, Bill Nye and Sean Carroll to be our witnesses and they graciously
01:09agreed.
01:10And Alex, I just want to say, I agree with everything you said about Veritasium, that I'm generally
01:17I can watch it and not have to wonder, is he going to mess up?
01:21What a hammer.
01:22No, he's brilliant.
01:23No, he's brilliant.
01:24I'm excited about this bet because if I am wrong, then I want to know.
01:28Like the whole point of the channel is to get to the truth and that is I think why we're
01:33all here today and I think, you know, this is a great, great chance to sort of see.
01:36I'm going to summarize Alex's main points in this video, but I'll put his full presentation
01:41here.
01:42So let me first explain what I see in the video.
01:46In the video, the vehicle is operated in a gusty wind.
01:51Initially, you have the wind speed exceeding the car speed, but then the wind speed is
01:56not constant.
01:57The wind speed drops and the car moves by inertia with deceleration for a while.
02:03So basically, Alex thinks a gust pushes the car to a high speed and then when the wind
02:08dies, the car is going faster than the wind momentarily, but it must be slowing down.
02:13In fact, that will be my conclusion at the end of this presentation, that whenever you
02:17have velocity faster than the wind, I'll actually show you in an equation, the acceleration
02:21is negative.
02:22The second effect is that the wind in the video is measured at the height of about a meter
02:29or a meter and a half, but the propeller goes to some, you know, three meters above the ground.
02:33Now due to interactions with the ground, there is a wind gradient.
02:37Wind travels slower, close to the ground, and then faster, higher up.
02:41Now, Alex estimated that the wind speed at the propeller might be 10 or 15% higher than
02:46at the Telltale.
02:47So it's possible that the car could be moving slower than the wind at the propeller and yet
02:51appear to be moving faster than the wind at the height of the Telltale.
02:55Now, I think that this is a small effect.
02:58However, in combination with the previous effect, it just can make this more frequent.
03:02Okay?
03:03And that completes.
03:04Yeah?
03:05If I remember the video correctly, Derek reports that they've achieved up to 2.8 times wind speed.
03:13That feels much higher than what is possible here unless the wind had picked up and then
03:19spontaneously sort of dropped.
03:21Very, very good question.
03:22Okay.
03:23If you're going for the record, you probably will do many attempts.
03:28You will be sampling that gust you wind over and over and over until you set the record.
03:33Right?
03:34That's how you set the record.
03:35And on one of those occasions, you will get a nice strong gust, which is three times the
03:40ebb that comes after it.
03:42Okay?
03:43And that's when you will clock the record and that's where that 2.8 factor will come in.
03:48So what about the treadmill tests?
03:50These are conducted in still air.
03:53By moving the ground backwards, you're simulating a perfectly steady tailwind.
03:58And if you hold the car stationary on the treadmill, well, that's equivalent to the car going exactly
04:04wind speed.
04:05Now, if the car can move forward on the treadmill, that shows it can accelerate faster than the
04:11wind.
04:13But Alex had multiple explanations why these experiments don't actually show what they
04:17claim to show.
04:19If you have this fluctuating speed of the treadmill, and then if a human just sort of steers it,
04:27it can introduce unconsciously a bias towards the desired result.
04:31So would it be that the guy with the spork is inducing the model craft to go across the
04:39wind now and then?
04:40And as you pointed out, Alex is going downhill now and then?
04:45Yeah, I'm sure.
04:46I'm 100% sure that the guy in the video doesn't do it on purpose.
04:52Okay?
04:53Yeah.
04:54Yeah.
04:55If he is expecting forward drift.
04:57Yeah.
04:58Oh, it's classic.
04:59Yeah, yeah.
05:00Absolutely.
05:01Exactly.
05:02Okay.
05:03So you got it.
05:04In the absence of convincing experimental evidence, Alex turned to theoretical analyses,
05:08like one by MIT aero professor Mark Drella.
05:11But here too, he found problems.
05:14His main concern is that the equation for net force includes the difference between the speeds
05:18of the car and the wind in the denominator.
05:22Which seems to imply that when traveling exactly wind speed, you should get infinite force.
05:27Now, here's a real danger.
05:29Because if Derek drives very close to the wind, that difference in speed goes to zero.
05:36If it's one millionth of a 1%, that's like a nuclear bomb exploding behind him.
05:41Then Derek is definitely in trouble, right?
05:43So we need to find something to save Derek's life here.
05:47This is serious, right?
05:48But dividing by zero, come on you guys, I never looked at that.
05:52Alex performed his own analysis and found no such divide by zero problems.
05:57In fact, he found there's no way for the cart to accelerate at or above wind speed.
06:03The acceleration of this craft is negative.
06:06So when we, you know, so it's possible to move the craft faster than the wind,
06:10but it's not possible to move it at zero acceleration, which would be needed to maintain constant speed.
06:16That is basically where we left it.
06:18Negative.
06:19Okay, thank you. Thank you, Neil. Thank you very much.
06:21Thanks, guys.
06:22Thanks, Neil.
06:23So now it was up to me to convince Professor Ksenko that Blackbird really can go faster than the wind.
06:29But his presentation was effective.
06:31When I posted about it on Twitter, Alice Zhang, who runs Chinese Veritasium said,
06:36I think you lost Derek. I'm 80% on Alex's side now.
06:40What's amazing to me is that neither one of them had seen my attempts to replicate the treadmill experiments.
06:45For the first video, I asked my friend and YouTube maker, Xyla Foxlin, to make a model downwind cart.
06:54Oh, no!
06:55Version one ended in failure, but Xyla was undeterred, coming back in a couple days with version two.
07:01Is it feeling like it's gonna...
07:03Unlike these models, most of her projects actually work. She is determined.
07:08So maybe this tells us something about whether you can actually go faster than the wind downwind.
07:14What was clear to me is that I didn't do a good enough job in the first video explaining how Blackbird works
07:20and providing convincing evidence that it can really go faster than the wind in a sustained way.
07:25In my defense, I thought the concept was well enough established.
07:29Way back in 1969, Andrew Bauer built the first successful downwind cart,
07:34and he did it to settle a friendly wager with aero engineer Apollo Smith.
07:39The bet was inspired by a claim in a student's paper from 20 years earlier.
07:44Now, Rick Cavallaro, the builder of Blackbird, was completely unaware of all this until after he built his cart.
07:49But other analyses have been published under names like the Push Me, Pull You boat.
07:54So I didn't honestly think anyone would doubt the vehicle's operation, much less bet me $10,000.
08:00But clearly there is a need for a deeper explanation.
08:03So I want to do that now by responding to the points Alex raised.
08:07So first, let's deal with wind gradient.
08:09I mean, why didn't we measure the speed of the wind higher up?
08:12Well, the answer is because it's already been done.
08:15They mounted Telltales on fishing poles out to the sides of the propeller and even above it.
08:20Now, although the lowest Telltale flips back first, all of the Telltales do eventually flip backwards,
08:26showing that every part of the vehicle is going faster than the wind.
08:31Could this be because of a big wind gust that pushed the car up to high speed and then the wind died?
08:36I don't think so.
08:38Even though I didn't have a speedometer in the car for my runs,
08:41someone on Twitter pointed out that we could use the rotation of the back wheel to determine the speed from the video footage.
08:47This shows that even after the Telltale flips backwards, the car keeps accelerating.
08:53Another thing I want to point out is that if wind gradient or gusts were the reason that the car travels faster than the wind,
09:00well, you'd expect the Telltale to jump around or at least not point straight back at me.
09:06But it consistently does for over 30 seconds until I had to hit the brakes to avoid crashing into parked vehicles.
09:12But 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 tailwind,
09:20it was still accelerating.
09:22And we know this because there were multiple GPS units in the car and wind speeds
09:27which were measured at the height of the propeller at multiple locations.
09:31The highlighted section shows the 10 second measurement period over which the record was set.
09:36Also, in 2013, the US Physics Olympiad semi-final exam asked questions about Blackbird,
09:42like can it go faster than the wind downwind and upwind.
09:47The solution says both modes are possible.
09:50And with sufficiently low energy loss, any speed is possible.
09:55Now I'll admit that the evidence I showed in the first video was not definitive.
10:00Wind gusts or gradients could have explained the observations.
10:04But now that you've seen this evidence,
10:06are you convinced that Blackbird can go downwind faster than the wind without slowing down?
10:11Well, Professor Ksenko was not convinced.
10:14So I want to explain how the car works so clearly that no one,
10:18not even the professor, can doubt what's going on.
10:21The first thing to know is that the propeller doesn't work like most people think.
10:25It's not working like a windmill.
10:27It doesn't turn the way the tailwind is pushing it.
10:30Instead, it turns in the opposite direction, working like a fan to push air backwards.
10:35This fan is powered by the wheels, which are connected to the propeller by a bike chain.
10:40So at wind speed, the car can keep accelerating because the wheels turn the fan,
10:44which blows air back, generating forward thrust.
10:48Now the big question is, to drive the fan, there must be a backwards force on the wheels,
10:53which tends to slow them down.
10:55So why isn't this force bigger than the thrust from the propeller causing the car to slow down overall?
11:01Well, the answer is, the wheels are going so much faster over the ground than the propeller is moving through the air,
11:08so the thrust force can actually be larger.
11:10I'm going to do an analysis in the frame of reference of the car.
11:14And the important equation to know is power equals force times velocity.
11:18So at the wheels, power is input into the system by the ground moving underneath the car.
11:24The power generated is the force of the ground on the wheels times the velocity of the car.
11:29At the propeller, work is done on the air as the propeller pushes it backwards.
11:35The power out equals the force of the prop on the air times the speed of the car minus the speed of the wind.
11:42The prop is going slower through the air due to the tailwind.
11:46And if we assume no losses, then the power in at the wheels equals the power out at the propeller.
11:52From this equation, we can see that the force at the propeller will be greater than the force at the wheels.
11:58And since the propeller is pushing air back, the air applies an equal and opposite force forward on the prop.
12:05This is the thrust force, which will be greater than the backwards force on the wheels.
12:12So this car works like a lever or a pulley.
12:16By applying a small force to the wheels over a larger distance, the propeller can apply a larger force over a smaller distance.
12:24This is just like when you're riding a bike going uphill.
12:27You move the pedals fast, but with smaller force, to make the wheels move slower over the ground, but with a bigger force.
12:34But now we've run into the divide by zero problem that Professor Kosenko warned us about.
12:39When the speed of the car is exactly equal to the speed of the wind, it seems like the propeller can provide infinite force.
12:46That can't be right, can it? I mean, is our analysis flawed?
12:50The answer is no, for two reasons.
12:53First of all, this is exactly what you'd expect theoretically with any lever or pulley.
12:58If one arm of the lever is zero, then you can lift an infinite weight with any amount of force on the other side.
13:04The catch is, its displacement will be zero.
13:08Second of all, in practice, there is a propeller efficiency term that is ill-defined when the propeller is not moving through the air.
13:15There's a better formula for the prop efficiency, which is well-defined in the zero airspeed limit.
13:21It makes an algebraic mess, but it's perfectly well-defined.
13:24And then the divide by zero problem is eliminated.
13:27But that equation makes the problem look more complicated than it actually is.
13:32You don't actually need aerodynamics.
13:34Here, I have a little cart with a big wheel that rolls on two smaller spools.
13:40And what I'm going to show is that when you have two media moving relative to one another,
13:46well then, if this car is in contact with both media, it can actually move faster than their relative velocity.
13:54So as I push the board to the right, you can see that the car goes down the board faster than the board is moving.
14:06If you look carefully, you'll see that the big wheel isn't turning the way that the board is pushing it.
14:13It's actually rotating in the opposite direction.
14:16That'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.
14:26Now you can build one of these cars for yourself at home, or you can build a model downwind cart.
14:33I told you Xyla was determined.
14:35Yeah, I'm going to make the claim on camera. I think it's going to work this time.
14:40We're changing the propeller.
14:41It has to work before we get kicked out of the treadmill store.
14:44Does it work?
14:55It totally works.
14:57Amazing.
14:58Oh my god, it's so good.
15:01Her fourth version of the cart works spectacularly, and it was designed to be replicated by anyone,
15:07using just a 3D printer and a simple list of materials.
15:10She explains how to build it with more details on the engineering process on a video over on her channel, so go check it out.
15:16Now, Professor Kosenko has now conceded the bet, and he transferred $10,000 to me.
15:22So I want to thank him for being a man of honor and changing his mind in light of the evidence I presented.
15:28Which is really not easy to do, especially in a public debate like this one.
15:32Now, I do not want to keep the money.
15:34I want to invest it in science communication.
15:37So I'm holding a one-minute video competition.
15:40I'll be awarding cash prizes to the top three videos that explain a counterintuitive STEM concept.
15:47I'll put some details down in the description.
15:50What I love about science is that disagreements are not problems.
15:55They are opportunities for everyone to learn something.
15:58I learned a lot more about Blackbird, aerodynamics, and gear ratios than I knew before.
16:04I also learned that I should go into more depth in my videos.
16:07I should make the evidence overwhelmingly convincing and put in some equations toward the end for those who want that level of detail.
16:13I want to thank everyone involved in the making of this video.
16:16Neil deGrasse, Tyson, Bill Nye, Sean Carroll, Mark Durella, Professor Kosenko, and Xyla Foxlin.
16:22But especially Rick Caballaro, the inventor and creator of Blackbird.
16:26He 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.
16:35Let's hope this video puts the issue to rest once and for all.
16:43The Blackbird craft all started with a brain teaser.
16:46And this video's sponsor, Brilliant, offers you a daily problem to solve every day.
16:51Like this one about gear ratios.
16:53If the first gear spins 10 times per second, at what rate does the final gear spin?
16:57Now, I did this problem the hard way, but my wife figured out how to do it the easy way.
17:01And that's what brain teasers are great for.
17:03They get you thinking about the world and they give you insights into problems you might think you already understand.
17:09Just 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.
17:17And while you're at it, why not take one of Brilliant's courses like on computer science, neural networks, or classical physics?
17:23Even physics professors can benefit from some lessons on frames of reference.
17:27Somehow, I managed to go my whole degree without learning Licorange in mechanics, so that's a course I'm working through at the moment.
17:33It's a really elegant way of solving physics problems, and I wish I had learned about it sooner.
17:39For viewers of this channel, Brilliant are offering 20% off an annual subscription to the first 200 people to sign up.
17:45Just go to brilliant.org slash Veritasium.
17:48I will put that link down in the description.
17:50So, I want to thank Brilliant for supporting Veritasium, and I want to thank you for watching.
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