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A Physics Prof Bet Me $10,000 I'm Wrong - YouTube
Channel: Veritasium
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I am here to sign a
document betting $10,000
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that my last video is in fact, correct.
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This is the video in question.
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Some people may have missed it,
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but in this car, there is
no motor, no batteries,
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no energy source, besides the wind itself.
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And the counterintuitive claim,
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is that this car can maintain speeds
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faster than the wind that's pushing it.
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There is a physics professor at UCLA,
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who got in touch, to say that
he thought that I was wrong,
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that the explanation was wrong.
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You know, we went back
and forth a little bit,
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and eventually I said,
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"Well, how about we bet $10,000?
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"I can prove it to you.
[41]
"This vehicle really can go
downwind faster than the wind."
[45]
And to my surprise, he is
taking me up on the bet.
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- Look, no one is perfect,
[50]
but you have a much lower error rate
[53]
than most people on
YouTube, in YouTube Space.
[56]
- Now, Professor Alex
Kusenko wanted this wager
[59]
and all related discourse to be public.
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In fact, he suggested we get a celebrity
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to witness the signing.
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So I asked Neil deGrasse Tyson, Bill Nye,
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and Sean Carroll to be our witnesses.
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And they graciously agreed.
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- And Alex, I just wanna say,
[72]
I agree with everything
you said about Veritasium.
[75]
That generally I can watch it
[78]
and not have to wonder,
is he gonna mess up?
[81]
How am I gonna-
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- No, he's brilliant, no, he's brilliant.
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- I'm excited about this bet,
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because if I am wrong, then I wanna know.
[88]
Like the whole point of the
channel is to get to the truth.
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And that is, I think why
we're all here today.
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And I think, you know,
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this is a great chance to sort of see.
[96]
I'm going to summarize Alex's
main points in this video,
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but I'll put his full presentation here.
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- So let me first explain
what I see in the video.
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In the video, the vehicle
is operated in a gusty wind.
[110]
Initially, you have the wind
speed exceeding the car speed,
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but then the wind speed is not constant.
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The wind speed drops, and
the car moves by inertia
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with deceleration for awhile.
[123]
- So, basically Alex thinks a gust
[125]
pushes the car to a high speed.
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And then when the wind dies,
[128]
the car is going faster
than the wind momentarily,
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but it must be slowing down.
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- In fact, that will be my conclusion
[134]
at the end of this presentation,
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that whenever you have
velocity faster than the wind,
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I'll actually show you in an equation,
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the acceleration is negative.
[142]
The second effect is that
the wind in the video
[146]
is measured at the height
[148]
of about a meter or a meter and a half.
[150]
But the propeller goes
to some three meters
[152]
above the ground.
[153]
- Now, due to interactions
with the ground,
[155]
there is a wind gradient.
[157]
Wind travels slower, close to the ground,
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and then faster, higher up.
[161]
Now, Alex estimated that the
wind speed of the propeller
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might be 10 or 15% higher
than at the tell-tale.
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So it's possible that
the car could be moving
[169]
slower than the wind at the propeller,
[171]
and yet appear to be
moving faster than the wind
[173]
at the height of the tell-tale.
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- Now, I think that
this is a small effect.
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However, in combination
with the previous effect,
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it just can make this more frequent, okay.
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And that completes-
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- Alex, if I remember the video correctly,
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Derek reports that they've achieved
[189]
up to 2.8 times wind speed.
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That feels much higher
than what is possible here,
[197]
unless the wind had picked up
[199]
and then spontaneously sort of dropped.
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- Very, very good question.
[202]
Okay, if you're going for the record,
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you probably will do many attempts.
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You will be sampling that gusty wind
[209]
over, and over, and over,
[212]
until you set the record, right?
[213]
That's how you set the record.
[214]
And on one of those occasions,
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you will get a nice strong gust,
[219]
which is three times the air
that comes after it, okay?
[223]
And that's when you will clock the record.
[225]
And that's where that
2.8 factor will come in.
[228]
- So what about the treadmill tests?
[230]
These are conducted in still air.
[233]
By moving the ground backwards,
[235]
you're simulating a
perfectly steady tailwind.
[238]
And if you hold the car
stationary on the treadmill,
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well, that's equivalent to the
car going exactly wind speed.
[245]
Now, if the car can move
forward on the treadmill,
[248]
that shows it can accelerate
faster than the wind.
[253]
But Alex had multiple explanations,
[254]
why these experiments don't actually show
[257]
what they claim to show.
[259]
- If you have this fluctuating
speed of the treadmill,
[264]
and then if a human
just sort of steers it,
[267]
it then can introduce unconsciously,
[269]
a bias towards the desired result.
[271]
- So, would it any be that
the guy with the spork
[275]
is inducing the model craft
[278]
to go across the lane now and then?
[280]
And as you pointed out, I
would go downhill now and then.
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- Yeah, I'm sure, I'm
a hundred percent sure
[287]
that the guy in the video
doesn't do it on purpose, okay.
[293]
However, you know, if he
is expecting forward drift-
[297]
- Yeah, oh he's trying.
[298]
- Yeah, yeah absolutely,
that's exactly, okay,
[301]
so there you go.
[302]
- In the absence of convincing
experimental evidence,
[305]
Alex turned to theoretical analyses,
[308]
like one by MIT Aero
Professor, Mark Drela.
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But here too, he found problems.
[314]
His main concern is that
the equation for net force
[316]
includes the difference
between the speeds of the car
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and the wind in the denominator.
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Which seems to imply that when
traveling exactly wind speed,
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you should get infinite force.
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- Now, here's the real danger.
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Because if Derek drives
very close to the wind,
[333]
that difference in speed goes to zero.
[336]
If it's one millionth of one percent,
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that's like a nuclear
bomb exploding behind him.
[341]
Then Derek is definitely
in trouble, right?
[343]
So we need to find something
to save Derek's life here.
[346]
This is serious, right?
[348]
- But dividing by zero, come on, you guys.
[350]
I never looked at that.
[351]
- [Derek] Alex performed his own analysis,
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and found no such divide by zero problems.
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In fact, he found there's
no way for the car
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to accelerate at, or above wind speed.
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- The acceleration of
this craft is negative.
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So when we, you know,
[368]
so it's possible to move the
craft faster than the wind,
[370]
but it's not possible to
move it at zero acceleration
[373]
which would be needed to
maintain constant speed.
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- [Derek] That is
basically where we left it.
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- That is right.
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- Okay, thank you, thank you, Neil.
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Thank you very much.
[381]
- Thanks guys, thanks Neil.
[382]
- So now it was up to me to
convince Professor Kusenko
[385]
that Blackbird really can
go faster than the wind,
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When I posted about it on Twitter,
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Alice Zhang, who runs
Chinese Veritasium, said,
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"I think you lost Derek.
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I'm 80% on Alex's side now."
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What's amazing to me, is
that neither one of them
[401]
had seen my attempts to replicate
the treadmill experiments.
[405]
For the first video, I asked
my friend and YouTube maker,
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Xyla Foxlin, to make
a model downwind cart.
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- All right.
(wheels screeching)
[412]
(laughing)
[414]
- [Man] Oh, no!
[415]
- Version one ended in failure,
[417]
but Xyla was undeterred,
[419]
coming back in a couple
of days with version two.
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- Is it feeling like it's gonna...?
[423]
- Unlike these models, most
of her projects actually work.
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She is determined.
[428]
So maybe this tells us something
[430]
about whether you can
[431]
actually go faster than
the wind, downwind.
[434]
What was clear to me is that
I didn't do a good enough job
[437]
in the first video,
explaining how Blackbird works
[440]
and providing convincing evidence
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that it can really go faster than the wind
[444]
in a sustained way.
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In my defense, I thought the concept
[447]
was well enough established.
[449]
Way back in 1969, Andrew Bauer
[451]
build the first successful downwind cart.
[454]
And he did it to settle a friendly wager
[457]
with Aero Engineer, Apolo Smith.
[459]
The bet was inspired by a
claim in a students paper
[462]
from 20 years earlier.
[464]
Now, Rick Cavallaro,
the builder of Blackbird
[465]
was completely unaware of all this
[467]
until after he built his cart.
[469]
But other analysis have been published
[471]
under names like the
push-me pull-you boat.
[474]
So I didn't honestly
think anyone would doubt
[476]
the vehicles operation,
much less bet me $10,000.
[480]
But clearly, there is a need
for a deeper explanation.
[484]
So I want to do that now
by responding to the points
[486]
Alex raised.
[487]
So first, let's deal with wind gradient.
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I mean, why didn't we measure
[491]
the speed of the wind higher up?
[492]
Well, the answer is because
it's already been done.
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They mounted tell-tales on fishing poles
[498]
out to the sides of the
propeller and even above it.
[501]
Now, although the lowest
tell-tale flips back first,
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all of the tell-tales do
eventually flip backwards
[507]
showing that every part of the vehicle
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is going faster than the wind.
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Could this be because of a big wind gust
[513]
that pushed the car up to high
speed and then the wind died?
[516]
I don't think so.
[518]
Even though I didn't have
a speedometer in the car
[520]
for my runs, someone
on Twitter pointed out
[522]
that we could use the
rotation of the back wheel
[525]
to determine the speed
from the video footage.
[527]
This shows that even after
the tell-tale flips backwards,
[530]
the car keeps accelerating.
[534]
Another thing I want to point out is that,
[535]
if wind gradient or gusts
[537]
were the reason that the car
travels faster than the wind,
[540]
well, you'd expect the
tell-tale to jump around
[542]
or at least not point straight back at me.
[545]
But it consistently
does for over 30 seconds
[548]
until I had to hit the brakes
[549]
to avoid crashing into parked vehicles.
[552]
But if that's not enough for you,
[553]
when Blackbird achieved its record speed
[555]
of 27.7 miles per hour in a
10 mile per hour tail wind,
[560]
it was still accelerating.
[562]
And we know this because
[563]
there were multiple GPS units
in the car and wind speeds
[567]
which were measured at the
height of the propeller
[569]
at multiple locations.
[571]
The highlighted section shows
[572]
the ten second measurement period
[574]
over which the record was set.
[576]
Also in 2013, the U.S Physics
Olympiad Semifinal Exam
[580]
asked questions about Blackbird.
[582]
Like, can it go faster than
the wind downwind and upwind?
[587]
The solution says, both
modes are possible.
[590]
And with sufficiently low energy loss,
[592]
any speed is possible.
[595]
Now, I'll admit that the evidence
[597]
I showed in the first
video was not definitive
[599]
when gusts or gradients
[601]
could have explained the observations.
[604]
But now that you've seen this evidence,
[606]
are you convinced that
Blackbird can go downwind
[608]
faster than the wind without slowing down?
[611]
Well, Professor Kusenko was not convinced.
[614]
So I wanna explain how
the car works so clearly
[617]
that no one, not even the professor,
[619]
can doubt what's going on.
[621]
The first thing to know,
[622]
is that the propeller doesn't
work like most people think.
[625]
It's not working like a windmill.
[626]
It doesn't turn the way
the tailwind is pushing it.
[630]
Instead, it turns in
the opposite direction,
[632]
working like a fan to push air backwards.
[635]
This fan is powered by the wheels,
[637]
which are connected to the
propeller by a bike chain.
[640]
So at wind speed, the
car can keep accelerating
[642]
because the wheels turn the
fan, which blows air back,
[646]
generating forward thrust.
[648]
Now the big question is, to drive the fan,
[650]
there must be a backwards
force on the wheels,
[653]
which tends to slow them down.
[655]
So why isn't this force bigger
[657]
than the thrust from the propeller
[659]
causing the car to slow down overall?
[661]
Well, the answer is,
[663]
the wheels are going so
much faster over the ground
[666]
than the propeller is
moving through the air.
[668]
So the thrust force
can actually be larger.
[670]
I'm gonna do an analysis
[672]
in the frame of reference of the car.
[674]
And the important equation to know is,
[676]
power equals force times velocity.
[678]
So at the wheels, power
is input into the system
[681]
by the ground moving underneath the car.
[684]
The power generated is the force
of the ground on the wheels
[687]
times the velocity of the car.
[690]
At the propeller, work is done on the air,
[693]
as the propeller pushes it backwards.
[695]
The power out equals the
force of the prop on the air,
[698]
times the speed of the car,
minus the speed of the wind.
[702]
The prop is going slower through the air
[704]
due to the tailwind.
[706]
And if we assume no losses,
[707]
then the power in at the wheels,
[709]
equals the power out at the propeller.
[711]
From this equation,
[712]
we can see that the force at the propeller
[714]
will be greater than
the force at the wheels.
[718]
And since the propeller
is pushing air back,
[720]
the air applies an
equal and opposite force
[723]
forward on the prop.
[725]
This is the thrust force,
which will be greater
[727]
than the backwards force on the wheels.
[732]
So, this car works like
a lever or a pulley
[735]
by applying a small force to the wheels
[737]
over a larger distance,
[739]
the propeller can apply a larger force
[742]
over a smaller distance.
[744]
This is just like when you're
riding a bike going up hill,
[747]
you move the pedals fast,
but with smaller force,
[750]
to make the wheels move
slower over the ground,
[752]
but with a bigger force.
[755]
But now we've run into the
divide by zero problem,
[757]
that Professor Kusenko warned us about.
[760]
When the speed of the car
[761]
is exactly equal to the speed of the wind,
[763]
it seems like the propeller
can provide infinite force.
[766]
That can't be right, can it?
[768]
I mean, is our analysis flawed?
[770]
The answer is no, for two reasons.
[773]
First of all, this is
exactly what you'd expect
[776]
theoretically, with any lever or pulley.
[778]
If one arm of the lever is zero,
[780]
then you can lift an infinite weight
[783]
with any amount of
force on the other side.
[785]
The catch is, it's
displacement will be zero.
[788]
But second of all, in practice,
[790]
there is a propeller efficiency term
[792]
that is ill-defined
[793]
when the propeller is not
moving through the air.
[795]
- There's a better formula
for the prop proficiency,
[798]
which is well-defined
in the zero speed limit.
[801]
It makes an algebraic mess,
[803]
but it's perfectly well-defined.
[805]
- And then the divide by
zero problem is eliminated.
[808]
But that equation makes the problem
[810]
look more complicated than it actually is.
[812]
You don't actually need aerodynamics.
[815]
Here, I have a little
cart with a big wheel
[817]
that rolls on two smaller spools.
[820]
And what I'm gonna show
[821]
is that when you have two media
[824]
moving relative to one another,
[826]
well then if this car is
in contact with both media,
[829]
it can actually move faster
than their relative velocity.
[835]
So as I push the board to the right,
[839]
you can see that the
car goes down the board
[842]
faster than the board is moving.
[846]
If you look carefully,
[847]
you'll see that the big wheel
[848]
isn't turning the way that
the board is pushing it.
[853]
It's actually rotating in
the opposite direction.
[856]
That's just like the
propeller on Blackbird,
[858]
which pushes back against the air,
[861]
and that's how it's able to go faster
[864]
than the wind downwind.
[866]
Now you can build one of these
cars for yourself at home,
[869]
or you can build a model downwind cart.
[874]
I told you, Xyla was determined.
[875]
Yeah, I'm gonna make the claim on camera.
[877]
I like, I think it's gonna work this time.
[880]
We're changing the propeller.
[881]
- It has to work
[882]
before we get kicked out of
the treadmill store. (laughing)
[884]
(motors roaring)
[893]
(chuckling)
[895]
- [Derek] Does it work?
[896]
- It totally works.
[897]
- [Derek] Amazing.
[898]
- Oh my god, it's so good.
[901]
- Her fourth version of the
cart works spectacularly
[904]
and it was designed to
be replicated by anyone
[907]
using just a 3D printer and
a simple list of materials.
[910]
She explains how to
build it with more detail
[912]
on the engineering process
[913]
on a video over on her channel.
[915]
So go check it out.
[916]
Now, professor Kusenko
has now conceded the bet,
[920]
and he transferred $10,000 to me.
[922]
So I wanna thank him for
being a man of honor,
[924]
and changing his mind in light
of the evidence I presented,
[928]
which is really not easy to do,
[929]
especially in a public
debate like this one.
[932]
Now I do not wanna keep the money.
[934]
I wanna invest it in
science communication.
[937]
So I'm holding a one-minute
video competition.
[940]
I'll be awarding cash prizes
to the top three videos
[944]
that explain a
counterintuitive STEM concept.
[947]
I'll put some details
down in the description.
[950]
What I love about science,
[952]
is that disagreements are not problems.
[955]
They are opportunities for
everyone to learn something.
[958]
I learned a lot more about
Blackbird aerodynamics,
[962]
and gear ratios than I knew before.
[964]
I also learned
[965]
that I should go into
more depth in my videos.
[967]
I should make the evidence
overwhelmingly convincing
[969]
and put in some equations toward the end
[971]
for those who want that level of detail.
[973]
I wanna thank everyone involved
in the making this video.
[976]
Neil deGrasse Tyson, Bill
Nye, Sean Carroll, Mark Drela,
[979]
Professor Kusenko and Xyla Foxlin,
[982]
but especially Rick Cavallaro,
[984]
the inventor and creator of Blackbird.
[986]
He was a fountain of information,
[988]
a constant source of support,
[990]
and the man leading the charge
to help people understand
[992]
this area of physics
for the past 15 years.
[995]
Let's hope this video
puts the issue to rest
[998]
once and for all.
[1000]
(electronics buzzing)
[1003]
The Blackbird craft all
started with a brain teaser.
[1006]
And this video sponsor, Brilliant,
[1008]
offers you a daily problem
to solve every day,
[1011]
like this one about gear ratios.
[1013]
If the first gear spins
10 times per second,
[1015]
at what rate does the final gear spin?
[1017]
Now I did this problem the hard way,
[1019]
but my wife figured out
how to do it the easy way.
[1021]
And that's what brain
teasers are great for.
[1023]
They get you thinking about the world,
[1025]
and they give you insights into problems
[1027]
you might think you already understand.
[1029]
Just think about
[1030]
how much more you would
understand in a year,
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if you got into the habit
[1034]
of solving one novel
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And while you're at it,
[1038]
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[1047]
Somehow I managed to go my whole degree
[1050]
without learning Lagrangian mechanics.
[1051]
So that's a course I'm
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It's a really elegant way
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And I wish I had learned about it sooner.
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For viewers of this channel,
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Just go to Brilliant.org/Veritasium.
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I will put that link
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[1070]
So, I wanna thank Brilliant
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[1073]
and I wanna thank you for watching.
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