00:00:00
This is me on top of a speeding train.
00:00:02
And this is me in a hot air balloon to answer some
of the most debated science questions on the internet.
00:00:07
For example, my hot air balloon pilot here says
he can land me anywhere I want, and yet there's
00:00:11
nothing here that resembles a steering wheel,
which begs the question, how do you actually steer
00:00:15
these things?
00:00:16
- So, we steer...
00:00:17
- And now I'm on top of the train to learn once
and for all, why do you land in the same spot if you
00:00:22
jump inside the train but not
if you jump on top of the train?
00:00:25
But I'm not stopping there.
00:00:27
(screaming)
00:00:28
- Because today we're gonna investigate 5 more
physics and engineering puzzles using simple
00:00:32
demonstrations as we go.
00:00:34
- Because our goal by the end of this video is not
for you just to know the right answers, but more
00:00:38
importantly for you to
understand why they're the right answers.
00:00:41
To kick things off, I've got a science
riddle for you that most people get wrong.
00:00:45
What is the first man-made
object to break the sound barrier?
00:00:48
- In other words, an object that travels more
than 767 MPH through the air, exceeding the speed of
00:00:54
sound, thereby creating a super loud sonic boom.
00:00:57
- So what do you think?
00:00:58
Most people I ask say either a
bullet or a rocket, or even a jet plane.
00:01:02
- But this tech goes way
back, like 5000 years way back.
00:01:06
- Because the first man-made
object to break the speed of sound
00:01:11
is a whip.
00:01:13
- The sound you just heard
was literally a sonic boom.
00:01:18
Which means at the peak of its motion
00:01:21
the tip of the whip is moving faster
00:01:25
than 767 MPH.
00:01:27
- OK, so if you got that
right, time for a bonus question.
00:01:30
What's almost surely the first
time you yourself heard a Sonic Boom?
00:01:34
- Here's your clue.
00:01:34
It's a near statistical certainty that everyone
has the same answer for this going back millions
00:01:40
of years.
00:01:40
- That's Right.
00:01:41
Thunder.
00:01:42
- Basically, the lightning superheats the air 5
times hotter than the surface of the sun, and this
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causes the air to expand so rapidly, it breaks
the sound barrier, once again creating a sonic boom.
00:01:52
- And if that juicy nugget of knowledge
just made your brain feel really good,
00:01:57
(cries)
00:01:58
Make that sound cooler in post.
00:02:01
Yeah.
00:02:02
- Buckle up because we've got 6
more to go, including at number 2...
00:02:06
- Have you ever noticed when grabbing drinks from a
cooler that even though they have the same amount
00:02:11
of liquid inside, regular
Coke sinks, yet Diet Coke floats.
00:02:15
Now why would that be?
00:02:16
Well, our first clue is that we know less
dense objects like this cork float and more dense
00:02:22
objects like this bolt sink.
00:02:23
However, that's not only true about different
objects, it's true about different liquids too.
00:02:28
Like for salad dressing, how the less
dense oil floats on the more dense vinegar.
00:02:33
And sure enough, when we compare the
respective labels, you'll notice one glaring difference.
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The regular Coke has more than 3 tablespoons of
real sugar, which makes this can more dense than
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the Diet Coke, which just
has artificial sweetener.
00:02:44
That means this one's just a little less dense
than water, causing it to float, while this one's
00:02:48
just a little more dense
than water, causing it to sink.
00:02:51
- But we can take this to the next level.
00:02:53
Here I have a wall that I filled with 8
different household liquids, from honey, to dish soap, to
00:02:58
baby oil.
00:02:59
And they naturally stack themselves
with increasing density as you go down.
00:03:03
So if we dump, let's say, a bunch of golf balls
in, they'll sink like a Coke can through all the
00:03:07
layers until they had a layer that's more dense
than they are, at which point the golf balls stop
00:03:12
falling and come to rest on top of that layer.
00:03:14
- Are you kidding me?
00:03:15
- Which means you can have a
different item come to rest on every layer.
00:03:19
- Whoa.
00:03:20
- As long as you spend way
more time than you care to admit,
00:03:23
- cool!
00:03:24
- testing way more items than you care to admit.
00:03:27
- This one's a dice roll.
00:03:28
- But in the end, it's not hard to
rationalize the wasted time when it looks this cool.
00:03:32
By the way, you can make a simple version of
this at home by filling a drinking glass with cooking
00:03:37
oil and dish soap, and then plopping
in a Lego guy to ride the ocean wave.
00:03:40
So this obviously works with household
liquids, but what if the liquid was metal like this?
00:03:45
- Because this bowl is 24 lbs...
00:03:49
of mercury.
00:03:50
It's hard to even submerge my hand in here.
00:03:53
Which means if I take an actual cast
iron anvil, it should float like a Diet Coke.
00:03:58
It's test it out.
00:04:00
(laughing)
00:04:02
Look at that.
00:04:03
I mean, I understand the science
behind this, but it still breaks your brain.
00:04:08
This is actual cast iron.
00:04:10
It's heavy.
00:04:11
And yet when you put it in
Mercury, it bobs like a cork.
00:04:14
Let's go bobbing for anvils.
00:04:16
- For number 3, a while back I read
about a famous mathematician from the 1600s.
00:04:20
He noticed that when he would hang up multiple
pendulum clocks in his house, they'd eventually
00:04:24
sync up their swings and
then they just stay that way.
00:04:27
So he wondered just like I did when I first read
this, how are these purely mechanical clocks that
00:04:31
weren't touching, somehow communicating with
each other and after a period of time, agreeing to
00:04:36
swing in perfect unison.
00:04:37
And since the unofficial motto of CrunchLabs
is think like an engineer, I decided to create an
00:04:42
experiment to test this for myself.
00:04:44
So we got 4 clocks and put them on the wall.
00:04:46
And after 4 days...
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nothing happened.
00:04:50
But when you're thinking like an engineer,
you know that's not a failure, that's a clue.
00:04:55
So I reread the story and picked up on an
important detail I'd missed the first time.
00:04:58
The clocks weren't mounted on a
wall in the way we think of today.
00:05:01
It said they were mounted on the same
wooden beam that made up part of the wall.
00:05:05
And given the construction methods of the
1600s, there's a good chance that board had a little
00:05:10
wiggle to it.
00:05:10
And that wiggle is a very important detail.
00:05:13
- Here's What I mean.
00:05:14
If I swing this sledgehammer pendulum below me
while standing on firm ground, then it's just the
00:05:19
sledgehammer pendulum that you see moving.
00:05:21
But if I stand on a skateboard where I don't
have firm footing, now you can see the pendulum is in
00:05:26
fact pushing back and moving me too.
00:05:28
Now, here's the wild part.
00:05:30
- Because the clock is securely attached to the beam,
that beam acts just a tiny bit like the skateboard.
00:05:35
So those swinging pendulums are ever
so slightly actually moving the wall.
00:05:39
So that means as the clocks are just slightly
pushing back and nudging the beam, they start to
00:05:44
influence and nudge each other too.
00:05:46
So if there's a clock that's greatly out of
sync with the others, over time, those little nudges
00:05:50
get it to line up.
00:05:51
So after discovering all this, we took those 4
clocks and this time attached them all to a beam,
00:05:55
which was attached to the wall
with a little bit of wiggle built in.
00:05:59
And this time after 4 days,
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once again, nothing happened.
00:06:03
Which once again was a clue, not a failure.
00:06:05
As it turns out, for this to actually work with
pendulum clocks, you need a very precise ratio of
00:06:11
clock to pendulum mass like this one set up here.
00:06:13
- So I couldn't get my hands on the perfect
pendulum clock, here's another way to observe the same
00:06:18
phenomenon with 140 pendulums all sitting on
a board that can freely wiggle back and forth.
00:06:23
All right, let's give it a shot.
00:06:24
Little help here.
00:06:32
- This is truly one of the
coolest things I've ever seen.
00:06:37
- It's like a bunch of stormtroopers.
00:06:41
- And just to give you a sense of how quickly they
sync up, here's an uninterrupted take so you can
00:06:46
see for yourself as the chaos
turns to order in less than a minute.
00:06:57
- So as you can see, even though we started
them totally random, in less than a minute, those
00:07:02
nudges do in fact influence each other till
they're all swinging together, just like the
00:07:07
clocks 400 years ago in the mathematician's house.
00:07:11
- Coming in at number 4, have you ever noticed every
time you see a full moon, it always looks the same.
00:07:15
In other words, it's the
same side, even in pictures.
00:07:18
Well, what about the other side?
00:07:20
- Well, it turns out we don't see the other side
because the moon is rotating perfectly in sync as
00:07:26
it orbits the Earth, so we
only ever see the googly eyes.
00:07:29
But that's wild.
00:07:31
Like, what are the chances
that those would sync up perfectly?
00:07:34
- Well, we call this tidal locking.
00:07:36
While it seems unlikely, you see this with pretty
much all the other planets and their moons in our
00:07:41
solar system too.
00:07:42
- And this simple demo will
give us the right mental model.
00:07:45
If the moon was perfectly spherical and balance,
it would spin like this, and one side wouldn't be
00:07:50
more likely to line up with Earth than the other.
00:07:53
But the moon's not perfectly spherical.
00:07:55
- Over time, that constant pull of Earth's
gravity has caused it to be sort of slightly oblong.
00:08:00
- Which we sort of exaggerated a little bit here,
00:08:03
but now you'll see since the center of gravity of
our moon doesn't exactly line up with the axis by
00:08:08
which it rotates like it was here,
00:08:10
now, no matter how I spin it, it always happens
to be that the heavy side of the moon always faces
00:08:16
the strongest source of gravity,
00:08:17
which just so happens to
be the center of the earth.
00:08:20
Well, guess what?
00:08:21
Out in space, the actual moon's nearest source
of gravity is still the center of the Earth, which
00:08:26
means that the real moon will always have the
same side pulled towards us, just like with our model.
00:08:32
And that means every living thing on this
planet that has ever looked up at the moon has always
00:08:37
seen that same familiar
face, and that will never change.
00:08:41
- Now, before we get to the last three,
including answering how hot air balloons steer,
00:08:45
if you're like me and you love the feeling of
that aha moment when you learn something new, then I
00:08:50
got great news for you because packaging
up that moment is why I created CrunchLabs,
00:08:53
- It's Mark Rober's engineering box!
00:08:56
- where you get a super fun toy every month in the
mail that comes with a video where I teach you all
00:09:01
the really cool physics that make it work.
00:09:02
So if you want to have a ton of fun building
your own toys while experiencing a bunch of those
00:09:07
lovely aha moments at the same time,
just visit crunchlabs.com to learn more.
00:09:12
- And now I'm on top of a train to learn once and
for all why do you land in the same spot if you
00:09:17
jump inside the train, but not
if you jump on top of the train.
00:09:20
- We'll get to the jump in a second, but first,
let's see if we can glean any clues from this
00:09:26
small scale test with two Lego guys, one inside
and one on top of the train, but they're in water.
00:09:32
Now in a moment, I'm gonna pull on this string,
which will move the train car forward, and I want
00:09:36
you to think what will
happen to each of the Lego guys.
00:09:39
Ready?
00:09:39
Here we go.
00:09:44
Did you catch that?
00:09:45
Now because this test was in water, you
intuitively sort of had a sense the top guy would
00:09:49
fall off and the bottom guy would be fine, right?
00:09:52
Well yeah, because the top guy would have to
travel through all that still water and that would
00:09:56
push back on him.
00:09:57
Whereas inside the train car, he's enclosed,
so everything in that car, including the water is
00:10:02
moving along with the train.
00:10:04
So why would I show you this Lego
guy standing on a model train in water?
00:10:08
- Because it's fundamentally no different
than me standing on a real train in air.
00:10:13
- Because just like water, air is also a fluid.
00:10:15
It's just not as dense as water.
00:10:17
You can sort of see how it moves like a fluid
here in this demonstration showing hot air coming from
00:10:21
a blow dryer.
00:10:22
So when a train moves through the
air, it's moving through a fluid.
00:10:25
To visualize this, let's replace
me with a pair of helium balloons.
00:10:29
Once again, what do you think will happen here?
00:10:31
Well, of course, the balloon inside is
totally upright, because just like in water, the air
00:10:36
around that balloon is traveling with the train.
00:10:39
Whereas on top, the balloon is being pushed
back because it has to fight its way through all the
00:10:43
stationary air, just like the Lego guy on top
being pushed back by all the water around the
00:10:48
train in the tank.
00:10:49
It's sort of like when you stick your hand out the
car window and feel all that resistance from the wind.
00:10:53
So going back to the jump, if you were able to
visualize all the air molecules like this, you'd
00:10:57
see how once we start moving inside the train when
I jump, I get no pushback from all my air molecule
00:11:01
neighbors because they're pretty
chill and we're all traveling together.
00:11:05
But on top of the train,
it's a whole different story.
00:11:07
I'm continuously bonking into a mad rush of
trillions of air molecules every second as I whizz by.
00:11:13
So if I jump now, it makes total sense why
they would push me back and I would land in a whole
00:11:18
different spot.
00:11:19
For number 6, here's an
interesting thought experiment.
00:11:21
Let's say you just bought a small
two-seater airplane and you know the plane has to be
00:11:25
traveling at 60 MPH on a still day to take off.
00:11:28
So one day you have the brilliant idea.
00:11:29
Why not just create a much shorter runway using a
plane sized treadmill that can ramp up to 60 MPH,
00:11:35
sending it into the air from your own backyard?
00:11:38
- But would that actually work?
00:11:40
Well, let's run the experiment
together with a small scale model.
00:11:43
- Now, the way a plane typically takes off is it
needs to reach a certain speed, which, of course,
00:11:47
then requires a certain length of runway.
00:11:49
So let's see what it is for this model airplane.
00:11:51
Looks like it's 10 miles an
hour after traveling 15 ft.
00:11:54
Now, here we've got our own version of our
backyard treadmill runway that will ramp up to our
00:11:59
10 MPH takeoff speed.
00:12:00
- So let's see what happens.
00:12:01
Make your prediction now if you haven't already.
00:12:03
- As we start the treadmill, we're gonna spin
the propeller more and more to keep it in the same
00:12:07
spot so it stays in the camera frame.
00:12:09
- OK, so we're at takeoff speed
and sadly, It's not taken off.
00:12:14
- At this point, we were suspicious
maybe we measured something wrong.
00:12:17
So we cranked it up way more.
00:12:19
And still nothing.
00:12:20
- So does that mean our
backyard treadmill idea is a bust?
00:12:23
- Let's talk about what needs to happen for a
plane to take off, because planes need air flowing
00:12:28
around their wings to create lift.
00:12:30
Once again, it's sort of like
sticking your hand out the car window.
00:12:32
You don't feel any lift stopped at a red light.
00:12:35
The faster you go, and the more air that
flows by your hand, the more lift you feel.
00:12:39
Which means no matter how fast our treadmill
goes, there's still no air moving over the wings.
00:12:44
We're stopped at a red light.
00:12:45
And I think part of the confusion many people
have with this experiment is they're comparing it to
00:12:49
the vehicle they know best, which is a car.
00:12:51
Because for a car, the ground is what gives
you forward motion, but for a plane, it's the
00:12:56
propeller that gives you forward motion.
00:12:58
Here's a thought experiment to demonstrate that.
00:13:00
Imagine you have a car and a plane
on the most slippery ice ever invented.
00:13:05
Because the car's engine uses the ground to get
forward motion through the tires, no matter how
00:13:10
fast you spin those tires,
the car will not move an inch.
00:13:13
The plane, however, is totally unaffected by the
ice, because the plane's wheels aren't powered at all.
00:13:19
The forward motion for a plane comes from the
propeller in the air, so it would take off with
00:13:24
absolutely no problems.
00:13:26
It wouldn't know the difference.
00:13:27
So knowing that, let's adjust this experiment
to give the plane a chance to move forward, which
00:13:32
means, of course, We're
gonna need a bigger treadmill.
00:13:35
So let's get our treadmill back
up to that 10 MPH takeoff speed,
00:13:39
- and at this point, we're exactly back to
where we started with the smaller treadmill.
00:13:43
- So to get the plane to move forward, let's just
give it more power, causing it to move down our
00:13:48
extended treadmill and
take off at 10 MPH after 15 ft.
00:13:52
- Which, as you recall, is the same speed and
distance as when we tested it with no treadmill at all.
00:13:58
- So would your brilliant
backyard runway treadmill idea work?
00:14:01
Absolutely.
00:14:02
Assuming, of course, your backyard is
the exact length of an actual runway.
00:14:06
And now for our final engineering puzzle, we're
back in this hot air balloon as I contend with my
00:14:11
mild fear of heights.
00:14:12
- OK, well, I haven't found a steering wheel, but
it does seem pretty clear every time you add hot
00:14:18
we go high.
00:14:19
- Hot air balloons utilize some pretty
straightforward physics, and the clue is right
00:14:23
there in the name.
00:14:24
Hot air is less dense than cold air, and we know
from our Coke cans, less dense things will rise.
00:14:30
So when Mateo does this and heats up the air
in the balloon, he's basically Diet Coking us.
00:14:35
And then over time, the air outside cools back
down the air in the balloon, making it more dense,
00:14:40
and therefore we start to
sink down like regular Coke.
00:14:43
- So all you have control over
Is whether we go up or down.
00:14:46
- Correct.
00:14:46
- But you said you could land me wherever I wanted.
00:14:49
- That is also correct.
00:14:50
- But what if the wind's going that
way and I want to land over there?
00:14:54
That's gotta be magic.
00:14:55
- Yeah, so what we'll do is we'll...
00:14:56
- OK, pause here, because to understand Mateo's
answer, you first need to understand two things.
00:15:00
The first is that if the wind is blowing in one
direction in your backyard, 1000 ft above that,
00:15:05
it's almost certainly blowing differently, and 1000
ft above that, it will once again be totally different.
00:15:11
So the question then becomes, how does Mateo know
the wind speed and direction at different heights?
00:15:15
And that's where number 2 comes in.
00:15:17
Because it's someone's job every day to launch
two balloons like this into the sky at noon and
00:15:22
midnight London time.
00:15:23
But this is done in 1000 locations all around
the world, all at those same two exact times.
00:15:28
And these balloons all have something called a
radiosonde attached to them that measures things
00:15:32
like altitude, pressure, temperature, and wind,
and then they transmit the information back to the
00:15:36
ground station, which gets fed into
supercomputers, and that's the reason the wind and
00:15:40
weather predictions can be so accurate.
00:15:42
So 2000 of these massive weather balloons go up
every day and they eventually pop and just land
00:15:47
somewhere, never to be retrieved.
00:15:49
Now, because of this, computers can figure out
the wind direction and speed at every level of the
00:15:54
atmosphere as you go up.
00:15:55
So on a given day, Mateo just checks the
daily charts, and if these are the predicted wind
00:15:59
directions and speed, and he wants us to
land exactly right here, he just needs to raise.
00:16:04
And then raise again.
00:16:05
And then lower the balloon to catch the
appropriate invisible rivers of air to land in the
00:16:10
perfect spot.
00:16:11
So hot air balloons don't steer, at
least not in the normal way we think of.
00:16:15
It's more of a tag team with Mother Nature.
00:16:17
So in some ways he's less of a magically
steering pilot and he's more of a weatherman.
00:16:22
- Which means the only real magic here is all
the juicy knowledge I just wirelessly transferred
00:16:27
through that screen from my brain into yours.
00:16:33
This is Timmy, and he is a piano prodigy.
00:16:37
He loves to practice and his parents
don't have to nag him to practice either.
00:16:41
I love it.
00:16:42
Do your thing, Timmy.
00:16:44
But this is Katie, and she lost all
interest in piano after Hot Cross Buns.
00:16:49
- Which means practice is a battle
and a chore she doesn't want to do.
00:16:52
And let's face it, mom
doesn't exactly want to hear either.
00:16:56
- Well, I got great news for you, Katie and mom,
because for the cost of one month of begrudging
00:17:01
piano lessons and reactions like this, you can
get an entire year of CrunchLab's Build Box with
00:17:06
reactions like this.
00:17:09
- And when the stoke level is this high, you're
learning so much cool stuff even without realizing
00:17:14
you're learning so much cool stuff.
00:17:17
Because each month when the new toy arrives on
your porch and then you put it together alongside
00:17:21
me, you're learning all the cool
engineering principles that make it work.
00:17:24
And this secret sauce of hiding the vegetables
is the reason we've shipped millions of these boxes
00:17:29
to converted parents around the world saying
this has totally unlocked a new level of resilience,
00:17:34
curiosity, and passion in their kids.
00:17:36
- And there's even two options.
00:17:38
You've got Build Box where I teach them the basics
through building a super fun toy for kids up to 12
00:17:42
years old.
00:17:43
And then there's Hack Pack where we learn some
coding with an awesome build it yourself robot for
00:17:48
your teens or heck, even for you.
00:17:50
- Half of these things get shipped to adults.
00:17:52
- So if you want to go from being the household
nagger to the household legend, watching a young
00:17:56
mind discover a passion they didn't even know
they had, go to CrunchLabs.com or use the link in the
00:18:01
video description where we're currently giving away
either 1 or 2 boxes free as an early spring special.
00:18:06
Thanks for watching.
