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Straw and water

  1. Oct 5, 2013 #1

    Check this video.
    How physic explains that? I know that it is because of atmospheric air,but I want more specific explanation.

    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Oct 5, 2013 #2
    The air pressure leads to a force on every surface, including the surface of the water inside the straw. Atmospheric pressure is ~10^5 Newtons per square meter, which is probably around 1 Newton for the cross sectional area of a straw. The weight of the water is less than 1 Newton, so the force from atmospheric pressure is sufficient to keep the water suspended in the straw.
  4. Oct 5, 2013 #3


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    it works for exactly the same reason you are able to suck liquid through a straw --- have you thought about that. I mean, if this puzzles you, why are you not puzzled that you can drink soda through a straw?
  5. Oct 5, 2013 #4


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    After the top of the straw is sealed, and as the straw is removed from the water, some of the water flows out the bottom of the straw, expanding and reducing the pressure of the air between the sealed top of the straw and the surface of the water inside the straw, and the pressure differential between the ambient pressure air outside the straw and the reduced pressure air within the straw generates enough upwards force to oppose gravity. The viscosity and the surface tension at the bottom of the straw "seals" the bottom of the straw well enough to prevent air from flowing up through the water inside the straw from below.
  6. Nov 2, 2013 #5
    okay, people who answered this, tell me this. what is the maximum possible height which is can hold using this method.
  7. Nov 2, 2013 #6


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    760 mm of mercury or about 30 feet of water.

    http://www.thermospokenhere.com/wp/02_tsh/B080___torricelli/barometer.png [Broken]
    Last edited by a moderator: May 6, 2017
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