Airplanes are held up by air pressure. Air is invisible, and it's thin. But sometimes it can be pretty strong. You can feel the pressure of air on your face when you walk into the wind or ride your bike. And air has pressure because it is made of something called molecules.

You've probably heard of molecules. They are the really tiny pieces of stuff that everything is made of. You, me, chocolate bars, and air are all made up of molecules. Molecules are always bouncing around, like kernels in a popcorn popper. They are always moving and vibrating. When air molecules bump into things, they make air pressure. Airplanes stay up in the air because of air pressure and a little something called Benoulli's (burnOOOH-leez) principle.

Bernoulli was a scientist who first realized that fast-moving air creates high air pressure

in the direction the air is moving.

But as air molecules move faster, they exert less pressure to the sides. You can feel this by blowing on the palm of one hand and bringing the index finger of your other hand close to your mouth. You feel strong air pressure in your palm, but almost nothing on the finger near your mouth.

Air exerts more pressure on things it runs into head-on&emdash;like your face, when you're riding a bike. And fast-moving air molecules exert less pressure on things they stream past&emdash;like the sides of your head.

Airplanes stay up in the air because of the shape of their wings and the angle that the air hits them. They are usually curved on top and sometimes they curve in underneath. The wing makes air molecules go slower under the wing than they do when they go over the wing. When the air molecules are going slowly, they bump up against the wing more, and push harder.

Bernoulli's principle is a way of saying that the faster air molecules moving over the top of an airplane wing push hardest in the direction that they're going. So they don't push down as hard as the slower air molecules going past the underside of the wing, which push up. So under the wings, there's higher pressure. That's how planes get "lifted" up. Pretty cool.

This idea of different pressures on opposite sides of things is what makes wings lift and baseballs curve. With big enough wings, a 40-ton 747 jet can stay up in the air, just by air pressure.

On the next page is an uplifting experiment you can try that shows air pressure in action.

Buoy, oh, Buoyancy

Why doesn't a heavy metal ship sink to the bottom of the ocean?

PLEASE CONSIDER THE FOLLOWING:

Ships do sink. They sink until they float. No kidding. An ocean liner doesn't rest on the surface of the water&emdash;it sinks a little bit and then floats along.

To better understand how giant ships float, let's imagine a smaller ship, like a rowboat. Just like its bigger cousins, when a rowboat sits in the water, part of the rowboat sinks under the surface. Since the surface of the water used to be flat, the boat must be pushing some water out of the way. Because the rowboat pushes water out of the place where the water used to be, scientists say the rowboat displaces water.

And we scientists know how much water the rowboat displaces. The rowboat displaces an amount of water that weighs exactly as much as the boat weighs. And that's true for every boat&emdash;big or small&emdash; on the water everywhere.

That's why an empty rowboat (or an empty oil tanker) floats higher in the water than a rowboat full of people (or a tanker full of oil). A rowboat with people in it weighs more than an empty rowboat. So it displaces more water than an empty rowboat. The boat plus the people weigh exactly as much as the water they're displacing . . . every time.

Ships float because water is pretty heavy stuff. See, an ocean liner weighs a lot and displaces a huge amount of water. But because it is hollow, the ship sinks only a little bit before it displaces an amount of water that's just as heavy as it is. That's why a solid metal ship wouldn't float. A ship made of solid steel would sink. It would displace plenty of water, but even after it displaced a ship- size amount of water, the ship would weigh more than the water it displaced. You know that solid metal things sink. It's because they can't ever displace an amount of water as heavy as they are.

The first guy to understand all this stuff and write it down was a man named Archimedes (ark- ih-MEE-deez). He got into a bathtub, it overflowed, and he suddenly realized that he had displaced water. He took up exactly as much room as the water he displaced. We take it for granted now, but two thousand years ago, it was a brand-new idea. The story goes that Archimedes was so excited by this discovery that he jumped out of his bathtub, shouted "eureka" (which means "I've found it"), and went running down the street with no clothes on! Hey, it was a big deal two thousand years ago, and it still is.

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Let's Sink This Chunk

You can see for yourself how a big ocean liner can sail the seas without sinking. All you need is some modeling clay and a bathtub.

• Mold some clay into the shape of a rowboat&emdash;it should look like a big clay bowl.

• Put your boat in a bathtub full of water.

• Does the boat sink or float?

• Now take your boat and squish it into a ball.

• Try floating the squished ball of clay.

• Does the ball sink or float?

When you put your clay boat in the water, it starts to sink and displaces water in a boat shape. Once the boat displaces water that weighs as much as the sunken part of the boat does, the boat stops sinking and starts floating.

The clay ball displaces water, too. But a clay ball isn't hollow like your boat. It starts to sink and keeps sinking. Eventually it displaces a ball of water the same size as the clay.

Since the clay ball weighs more than the ball of water, the clay sinks to the bottom.