|MadSci Network: Physics|
Consider a glass of water as a container that has a very large hole in the top. Why doesn't the water jump up out of the glass? Well, there's two ways to look at it: 1) Remember there's gravity pulling downward on the water. And gravity is a *vector* field - that means it has a *direction*. Gravity pulls *down* on it, so the molecules are exerting force *down*. Consider the molecules at the surface - gravity is pulling them down, so they exert a force downward on the layer of molecules under them. And that second layer exerts force of its own, *and* propagates the force from above, and so on. The end result is that at the surface, there's zero up/down pressure. 32 feet down, there's enough water above you to equal the weight of atmosphere above you, so you have about 28 pounds/square inch of pressure total, and so on - at 320 feet, you have 11 atmosphere's worth of things pressing down. 2) The water *is* exerting a small upward force caused by (mostly) the fact it isn't very compressible, which just happens to perfectly balanced by the *downward* pressure of many miles of atmosphere above it. To demonstrate what happens when there's an imbalance, use a straw to either blow air into a concentrated spot on the surface and watch the added pressure cause a small dip in the water surface. Or suck on the straw, and watch the imbalance cause the water to rise up the straw. This force is quite strong - about 14 pounds per square inch, which over the entire surface of the glass is quite a bit pushing down on the water, keeping it in place. In fact, if you fill a glass *almost* full of water, then place a small *stiff* piece of cardboard or plastic across the top, and then invert the glass while holding the cover in place, you can hold the glass *upside down* and the air pressure will keep the cover in place. Just two notes: (a) practice over a sink and (b) a *totally* full glass won't work as well. And yes, some of the molecules *do* exert enough force upwards to leave the surface. The usual term for that is "evaporation".
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