[MUSIC] This episode is sponsored by Dropbox [MUSIC] If it sometimes feels like the whole world
is pressing in on you, well that’s because it is.
A 1 square meter column of air above your head, stretching from here to the edge of
Earth’s atmosphere contains almost 10 tons of air. That’s a lot to carry on your
shoulders. But if it ever feels like too much, you know
what’s a great way to get away from it all? Balloons! That’s not what I meant guys. No, not like that, like this. [MUSIC] Balloons are magical.
Birds and planes stay aloft by using power to keep air moving over their wings [HELIUM VOICE]
We fill blimps and party balloons with lighter gases like helium.
But balloons like those weigh thousands of pounds, and they’re not filled with helium,
they’re filled with the very same air they’re floating in, yet they still go up.
Now that you mention it, why does the atmosphere itself stay up? That column of air above your head we talked
about earlier? It’s the same mass as a couple of elephants!
But just like elephants, air molecules obey gravity, and I don’t see any floating pachyderms
around. Think about it. Air is mostly empty space.
If I was an oxygen molecule, I’d have to travel the length of a football field on average
before I ran into another one. But occasionally, they do collide. Air molecules will fall until the collisions
bouncing them up, the pressure, is balanced by the weight pushing down on them.
Up high in the atmosphere, there’s not much air on top of us, and not much colliding going
on, so the pressure is pretty low. Down a little lower, we have a big mass of
air above squishing down, so things get a bit more crowded and the pressure is higher.
This sets up a gradient all the way down to Earth.
The lower we go, the air is more crowded, it takes more collisions to balance out that
squishing. Even though it’s mostly empty space, air
can still hold up a plane. I can sit in a chair, in the sky, riding on
almost nothing, and not fall to the ground. That is crazy. You may have heard that the Wright Brothers
are responsible for humans taking flight. But that’s wrong. 120 years before them,
in 1783, the Montgolfier brothers had the brilliant and kind of dangerous idea to light
a fire underneath a cloth b Heat ius what’s keeping this balloon up.
You’ve been told your whole life that hot air rises, but have you ever REALLY stopped
to ask yourself why? You know what? We’re gonna need a smaller balloon to figure that
one out. Ah, that’s better.
This is the ideal gas law, and it tells us pretty much everything we need to know about
how gases behave. Pressure is basically the force over a given
area on a surface. Whether a balloon inflates depends on the difference in pressure inside
and out. As we heat up air in a balloon, the molecules
gain kinetic energy and start colliding with more force on the inside surface. The pressure
goes up. As the balloon expands, that force is spread
across a larger surface area, which lowers the inner pressure. As we add heat, the volume
will go up up until the pressure on both sides is the same. Or until it pops. But this balloon isn’t made out of rubber.
It’s pretty rigid, once it fills up, it can’t really get bigger. But it does have
one thing that the rubber balloon doesn’t. A hole in the bottom. You know what the mass of air inside the balloon
envelope is? It’s 105,000 cubic feet. We can calculate that with a little bit of
math. It’s 105,000 cubic feet. I don’t know to be
honest. Well, I calculated it… When we first filled up this balloon on the
ground, it held about 3700 kg of air. Add in the basket, the fuel, the people, and of
course the balloon itself, and’s that’s another 500 kg or so. A lot of mass to lift. But then we added some… FIIIIIIIRE! [HEAVY METAL MUSIC] As we’re adding heat to this balloon, those
air molecules are moving around faster and faster, with more energy, and they’re pushing
on the balloon harder and harder, but the volume has remained the same, it hasn’t gotten
any bigger or smaller. The pressure is pretty much the same too, the balloon’s not collapsing
or popping. So that only leaves us with one other option,
there must be less air inside the balloon than when we were down on the ground. Hot
air has leaked out the bottom. We’re still being pulled down by gravity,
but the equal volume of cold air around us weighs more, and it’s falling down underneath
us. Balloons don’t defy gravity, they float because of gravity. Eureka! This is the principle of buoyancy
that Archimedes noticed when he saw his bathtub rise. It’s the reason that ships float,
because the weight they hold is less than the weight they displace, and the water falls
down around them. We know that buoyancy is due to gravity, because
bubbles rise here on Earth, and they don’t rise on the Space Station. You said that flying in a balloon is like
what? Flying in a hot air balloon is the closes
thing to being a cloud that I can think of. We’re just floating with the wind, as gentle
as can be. Flying in a hot air balloon kind of reminds
me that we have something in common with fish. We live in a fluid, it’s just that ours
is made of air instead of water. It’s kind of like floating on a bubble, and
that’s something that can lift all our spirits. Stay curious.