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Bottomless fish tank pressure brainteaser

  1. Nov 1, 2013 #1
    Link at the bottom if you do not know what I am talking about. Fish tank that works like pet watering bowl.

    If you poked a hole at the top air would go rush in.
    If you poked a hole at the bottom water would rush out.

    At what point in the tank would water be under so little pressure, the tension would be enough for it to keep from leaking?

    http://www.okeanosgroup.com/blog/aquariums/fun-with-physics-bottomless-fish-tanks/ [Broken]

    This is a brain teaser but I don't have the answer yet.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 1, 2013 #2


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    Science Advisor
    Gold Member

    I am not sure exactly what you are asking. Surface tension doesn't play a role here. It is all about pressure. The surface of the water outside the pillar-type tank is under atmospheric pressure of 101325 Pa (14.7 psi). At that same level inside the vertical tank, there is no air, so it is only under the pressure from the water above. If it was going to fall down out of the vertical tank and thus raise the surface level outside of the tank, it would have to exert more pressure at the level of the surface than the air does, so conceivably, you could have a column of water up to 10 m tall before the tank would empty itself.

    If you poke a hole in the top, no amount of surface tension will save you. That water will fall right out and overflow the bottom tank.
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