Kind: captions
Language: en
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 no one
is perfect uh but uh you have a much
lower error rate than most uh people on
YouTube in YouTube space now Professor
Alex cenko wanted this wager and all
related course to be public in fact he
suggested we get a celebrity to witness
the signing so I asked Neil degrass
Tyson Bill Nye and Shan Caroll to be our
Witnesses and they graciously agreed and
and Alex I just want to say I agree with
everything you said about
veritasium that I'm generally I can
watch it and not have to wonder is he
going to mess up what how am I no he's
brilliant brilliant I'm excited about
this bet because if I am wrong then I
want to 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 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 a while 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 uh the
acceleration is negative okay the second
effect is that the wind in the in the
video is measured at the height of about
a meter or meter and a half but the
propeller goes to some you know 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
at the propeller might be 10 or 15%
higher than at The Telltale 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 Telltale
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 um if I
remember the video correctly D 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 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 E that comes after it
okay and that's when you will clock the
record uh 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
windspeed 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 uh this
uh you know fluctuating speed of the
threadmill and then if a human just sort
of steers it it can introduce
unconsciously a b towards the desired
result so what it may be that the guy
with the spork is inducing the model
craft to go across the wind now and then
so and as you pointed out Alex going
downhill now and then yeah I'm sure I'm
100% sure that uh the guy in the in the
video doesn't do it um on purpose okay
yeah however you know if he is expecting
uh forward drift yeah oh it's classic
yeah yeah absolutely that that's that's
what exactly okay so you got in the
absence of convincing experimental
evidence Alex turned to theoretical
analyses like one by MIT AO Professor
Mark drella 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 winds speed you should get
infinite Force now now here's a real
danger because if if if DK drives very
close to the wind that difference in
speed goes to zero if it's 1 millionth
of a 1% that's like a nuclear bomb
exploding behind him then Derek is
definitely in trouble right so we need
we need to find something to save
Derek's life here this is this is
serious right but dividing by zero come
on you guys I never looked at that Alex
performed his own analysis and found no
such divide by zero problems in fact he
found there's no way for the cart 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 that is
basically where we left it okay thank
you thank you Neil thank you very much
guys thanks Neil so now it was up to me
to convince Professor cenko that
Blackbird really can go faster than the
wind but his presentation was effective
when I posted about it on Twitter Alice
Zang 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 zyla foxin to make a
model downwind
cart oh no version one ended in Failure
but zyla was undeterred coming back in a
couple days with version two is it
feeling like it's going to unlike these
models most of her projects actually
work she is detered 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 Bower built the
first successful downwind cart and he
did it to settle a friendly wager with
Aero engineer Apollo Smith the BET was
inspired by a claim in a student's paper
from 20 years earlier now Rick cavalero
the Builder of Blackbird was completely
unaware of all this until after he built
his cart but other analyses have been
published under names like the push me
pull you boat so I didn't honestly think
anyone would doubt the vehicle's
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 taals on fishing poles out
to the sides of the propeller and even
above it now although the lowest
Telltale flips back first all of The
Telltale 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 Telltale 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 wind well
you'd expect The Telltale 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 mph in a 10 mph Tailwind 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 10-second measurement
period over which the record was set
also in 2013 the US physics Olympia
semi-final 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 wind gusts or gradients
could have explained the observations
but now that you've seen this evidence
are you convinced that blackb bird can
go downwind faster than the wind without
slowing down well Professor cenko was
not convinced so I want to 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 going to do an
analysis in the frame of reference of
the car and the important equation to
know is power equals force time 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 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 uphill
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
by0 problem that Professor cenko 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 its displacement will
be zero second of all in practice there
is a propeller efficiency term that is
il defined when the propeller is not
moving through the air there's a better
formula for the prop efficiency which is
well defined in the zero air speed limit
it 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 going to
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
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 zyla was determined y I'm I'm
going to make the claim on camera I like
I think it's going to work this time
we're changing the propeller it has to
work before we get kicked out of the
treadmill
store does it work it totally works
amazing oh my 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
details on the Engineering Process on a
video over on her channel so go check it
out now Professor cenko has now conceded
the BET and he transferred $10,000 to me
so I want to 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
want to keep the money I want to 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 want to thank everyone
involved in the making of this video
Neil degrass Tyson Bill and NY Shan
Caroll Mark drella professor cenko and
zyla foxlin but especially Rick cavalaro
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 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 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 lessons
on frames of reference somehow I managed
to go my whole degree without learning
lanji in 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 want to thank
brilliant for supporting veritasium and
I want to thank you for watching