Physics Question #9010

Kyler Tonkin, a 15 year old male from Kingston, Ontario asks on December 16, 2013,

I heard of an experiment that it is impossible to suck a liquid up a straw more than 10 meters. Is this a true experiment?

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The answer

Barry Shell answered on December 17, 2013

Yes it's true. This has to do with the pressure of air at the surface of the earth. When water is "sucked" up a straw, it's not only your sucking that makes it come up, it's the pressure of all the air around you that does it. Everywhere on Earth there is a blanket of about 10 - 20km of air all pressing down on the surface of the planet and it's pressing on the container of water that's holding your straw. When you suck a bit of air out of the straw, all that air on the outside presses down on the water and pushes it up the straw. The most you can suck out of the straw in an ideal case is EVERYTHING--which creates a total vacuum in the straw. In that case the atmosphere presses down the maximum amount on the water and pushes it up the straw. This maximum pressure is equal to the air pressure at the surface of the earth, which on average is enough to push about 10m of water up a tube. After that, there's not enough air pressure to push it any higher. You can find this answer all over the internet. An even more interesting question is: if this is true--that 10m is the maximum height that water can be sucked up a tube, then how can trees that are over 100m tall "suck" up water all the way to the leaves at the top? The answer is here: http://www.science4all.org/le-nguyen-hoang/the-amazing-physics-of-water-in-trees. If you watch that video you will have much greater respect for the power of trees, and the laws of physics.

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