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Hydrostatic and atmospheric pressure

  1. Jul 4, 2015 #1
    So I've been wondering, 10.3 meter of water amount for one atmosphere, and according to Pascal's law pressure can compute by P=P0+ ρgh. If we have a glass with 10 cm of water inside , the pressure in the glass would be lower than atmospheric pressure and therefore when you turn the cup upside down water shouldn't fall, like when you have a straw! But in reality water does fall, what am i missing ?

    thank you for your help :)
     
  2. jcsd
  3. Jul 4, 2015 #2
    The pressure under any depth of water is the atmospheric pressure (P0) above the water plus the rho g h from the static water pressure.
     
  4. Jul 4, 2015 #3
    I expect that i have a vaccum in the bottom of the glass, so P0=0.
     
  5. Jul 4, 2015 #4
    P0 is the pressure at the top, not at the bottom.
     
  6. Jul 4, 2015 #5
    I'm talking when you turn the glass upside down.... pressure at bottom should be 0, right? pressure at the surface should be equal to 1 atmosphere.
     
  7. Jul 4, 2015 #6
    My question could be formulated in a different way. If we can hold 10.3 meters of water in a straw, why doesn't the same happen with the glass?
     
  8. Jul 4, 2015 #7

    russ_watters

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    Staff: Mentor

    Water is a liquid and can't hold its shape well enough to stay in. Try the same with a can of jello.
     
  9. Jul 4, 2015 #8
    So that is the reason...Thank you very much!. In the straw water holds because it's somehow easy for water to maintain it's structure, right ?
     
  10. Jul 4, 2015 #9

    russ_watters

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    Staff: Mentor

    Yes. For the small diameter of the straw, the surface tension of the water is enough to hold it's shape.
     
  11. Jul 7, 2015 #10
    firstly inverting the glass you have p atm at the open surface of the glass trying to push the liquid inwards BUT
    oh man you are forgetting mg the weight of water , the pressure cannot balance that !!
     
  12. Jul 7, 2015 #11
    No, you're wrong. Pressure does balance that. That's why if you put a card on top of the glass and turn it down, the card would stuck onto the cup as well as the water . Pressure is a manifestation of the weight of a substance on other. I'm pretty sure i'm right, nevertheless it will be good if someone could back me up.
     
  13. Jul 7, 2015 #12
    hey
    the card is so light !!
    however we all know that water falls off the glass, so then how can you reason that ???
    if i am wrong
     
  14. Jul 8, 2015 #13
    I could be explaining in words, but this video does a better job :
     
  15. Jul 9, 2015 #14
    i am unable to open the video link
     
  16. Jul 9, 2015 #15
    please send the URL
     
  17. Jul 9, 2015 #16
    Go to youtube search for : The history of the barometer (and how it works) - Asaf Bar-Yosef
     
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