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Q & A: Air and Water... Which is Stronger?

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Q:
Is air pressure stronger than water pressure
- nicole jackson (age 12)
brooklyn,ny,usa
A:
Nicole -

Actually, water pressure is generally stronger than air pressure. Think of it this way... if you had a plastic bottle filled with water and you poked a hole in the side, would water squirt out or would air squirt in? If air squirted in, then the air pressure would have to be higher. But since we know that the water would squirt out, we know that the water’s pressure must be stronger. Of course, this is under normal conditions. If you pumped the air pressure up enough (like you do when you pump air into a bike tire), then the air pressure may become stronger than standard water pressure.

-Tamara

To be precise- see below.

(republished on 08/02/06)

Follow-Up #1: water and air pressure

Q:
At sea level air pressure is actually stronger than water pressure. Tamara, with your bottle example, if you take a 2 liter bottle and fill it with water, cap it, then poke a hole in it, no water will come in. If you open the cap and allow the air pressure in from the top the water will squirt out of the bottle, because air pressure is pushing down from the top. Put the cap back on and the flow of water stops, because the air can no longer push from the top.
- Shirley
Anaheim, CA USA
A:
I'm pretty confused by both questions and the previous answer.

If you look at the surface between some water and air, say in a lake, it's not getting pushed either way. The net force on it is zero. That means that the pressure pushing up from the water has to exactly equal the pressure pushing down from the air. (The surface doesn't supply any 'surface tension' force unless it's bulging one way or the other.)

What happens if you look above or below the surface? The pressure difference between the bottom and top of some region has to be big enough to counteract the force of gravity, otherwise the fluid would fall. Air isn't very dense, it doesn't weigh much per volume. So the pressure falls off slowly as you go up. You have to go up a few kilometers before it drops to half the surface value. Water is much denser, so the pressure goes up faster as you go down. At 10 meters below the surface, the pressure is already about twice the surface pressure.

The closed bottle example is trickier because there are two different air pressures involved, above the water in the bottle and outside the bottle. Even the open bottle example is tricky because there are different water pressures, ranging from the same as air pressure (at the top) to a little higher (at the bottom).

Mike W.



(published on 06/18/09)

Follow-Up #2: one bar of pressure

Q:
Is one bar of air the same as one bar of water
- Mark Savage (age 46)
UK
A:
The pressure unit, "bar", refers to the same pressure regardless of the material. If you had a piston with one water at one bar on one side and air at one bar on the other, it wouldn't get a net push either way.

Mike W.

(published on 07/19/11)

Follow-up on this answer.