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if you submerge a glass under water and pull up, how does the water come up in the cup? (I believe its air pressure, but from where since all of the air is removed when it is submerged?)
The water is held in the glass by air pressure from the air outside of the glass
When you lift the glass up, the water inside starts to fall down. This
leaves a small space at the top of the glass with no water and no air
in it (a vacuum). Air outside of the glass pushes down on the water in
the container, pushing it back up into that space. Since there's no air
there to push it back down, it stays there.
If you did this with a tall enough glass, the force of gravity
would eventually be enough to pull the water down part-way, leaving a
visible vacuum-filled space at the top. It is actually possible to
build a simple barometer (a device that measures air pressure) using
this idea. The higher the air pressure, the farther up the glass the
water will be pushed. More often, though, this sort of barometer is
made using liquid mercury instead of water, since you can get the same
results using a much shorter tube of mercury than if you were using
(published on 10/22/2007)
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