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This episode is sponsored by Dropbox
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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.
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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.