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  • You may not realize it,

  • but from the moment you got out of bed today

  • to the point where you sat down to watch this video,

  • you've essentially been swimming.

  • Why? Because air is a fluid just like water.

  • It has waves and eddies.

  • It flows.

  • And when you push air out of the way,

  • it rushes around you into a wake.

  • So why don't we notice it most of the time?

  • We commonly think of air as empty space.

  • But while one cubic centimeter of interstellar space,

  • the volume in the tip of your pinky finger,

  • contains roughly one atom,

  • the same volume of air has about 10 quintillion molecules.

  • If that sounds hard to wrap your head around,

  • it happens to be about the same

  • as the number of insects alive on the planet,

  • all crawling, climbing, and flying over each other

  • in an enormous, tightly packed swarm.

  • When this swarm of molecules runs into things,

  • it exerts a force, pressing against the boundaries of the fluid,

  • like water pressing against the glass of a bottle.

  • This is known as air pressure.

  • And while air is lighter than water,

  • all those molecules still get pretty heavy,

  • with the total air filling a typical school gym,

  • weighing about as much as an adult elephant.

  • So when you walk into a gym,

  • how come you're not immediately crushed

  • by the elephant of air in the room?

  • Well, first of all,

  • because most of it is pressing on the floor and the walls,

  • and the part that is pressing on you

  • is pushed back by the pressure inside you!

  • You see, the air, as well as the water and everything else,

  • that fills our bodies exerts an amount of pressure

  • equal to that of the air outside.

  • Of course, this is no accident.

  • It's precisely what allows us to survive

  • in the normal atmosphere,

  • and what makes it more difficult

  • at high altitudes or deep water.

  • And we normally don't feel the air pressing on us

  • because it's generally uniform.

  • So even though different amounts of air molecules

  • are hitting you at different times,

  • the swarm is so thick

  • that all those little differences average out.

  • What happens when air pressure isn't uniform?

  • This means that the molecules are pushing harder

  • in one region of air than another,

  • driving the air flow from the higher pressure region

  • to the lower.

  • We feel this flow directly as wind,

  • and the pressure systems

  • that meteorologists are always going on about

  • are responsible for other weather changes,

  • from the mundane to the catastrophic.

  • But differences in air pressure

  • do more than just let us complain about the weather;

  • they're the very reason we're alive.

  • We breathe by lowering the pressure in our lungs,

  • allowing air to rush in.

  • So the next time you take a deep breath,

  • think of the unfathomable number of air molecules

  • you're commanding to move.

  • We look up at the night sky

  • to ponder the infinity of space,

  • but unless you're watching this video

  • from that deep space,

  • there are more air molecules

  • in and around your body

  • than there are grains of sand in all the world's beaches and deserts,

  • stars in the visible universe,

  • or both of those numbers combined.

  • The vastness of the universe

  • is right in front of you

  • and inside you.

You may not realize it,

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B1 US TED-Ed air pressure pressing air pressure swarm

【TED-Ed】How heavy is air? - Dan Quinn

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    Ashley Chen posted on 2014/07/11
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