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Q & A: Upside-Down Water Glass

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Most recent answer: 10/22/2007
Q:
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?)
- Anonymous
knoxville,tn,usa
A:
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 water.

-Tamara

(published on 10/22/2007)

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