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Water in upside down cup

  1. Aug 22, 2005 #1
    In grade school I remember the physics experiment of the cup of water turned upside down on a card, and kept the water in the cup, my question is based on the same principle except that the cup is turned upside down in a bigger container of water, my question is , is the water in the cup at the same pressure as in the bigger container of water, or is it slightly different?
    If it is slightly different , is the pressure in the cup any different from the top of the cup compared to the bottom of the cup?
    Also if anyone knows a good link on this , I'd appreciate a heads up, Thanks
     
  2. jcsd
  3. Aug 22, 2005 #2

    Tide

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    If the pressures were the same then there would be no net pressure force on the water in the glass and the force of gravity would be "unchallenged."
     
  4. Aug 23, 2005 #3

    SGT

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    Water pressure grows from the surface to the bottom.
     
  5. Aug 23, 2005 #4

    Clausius2

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    Well, if you turn upside down a cup of water on the top of a big container of water, then the water escapes out the cup and it becomes empty. Am I loosing something?.
     
  6. Aug 23, 2005 #5
    When the cup has water in it and is lifted upside down in say a pan of water, as long as the cup doesn't break the field of the pan water and let in air, it keeps the water at the highest level of the cup because of the vacuum, thats what was meant.
     
  7. Aug 23, 2005 #6

    Clausius2

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    I see. Employ Hidrostatics. If the cup lenght is small enough, there won't be approximately any pressure difference.
     
  8. Aug 27, 2005 #7
    Tide says, If the pressures were the same then there would be no net pressure force on the water in the glass and the force of gravity would be "unchallenged."
    But wouldn't it be held in the glass by vacuum and not pressure, thus elimiting the build up of pressure?
     
  9. Aug 28, 2005 #8

    Tide

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    Lock,

    The air pressure inside the glass is not zero so it has to exert a downward force on the water. Likewise, the air below exerts an upward force on the water. The net pressure force is the sum of those forces (one negative and the other positive) and must be nonzero in order to sustain the weight of the water.
     
  10. Aug 29, 2005 #9

    Gir

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    the problem is air pressure not water pressure. as long as air cannot get in, the pressure inside will not change.
     
  11. Aug 29, 2005 #10

    lightgrav

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    Tide is refering to the pressure at the water surface
    just under the glass "bottom" (which is now on top) as "air pressure".
    You "always" get a small amount of air inside the glass.

    The pressure at higher elevations is always less than at lower,
    unless there are drastic changes in the speed (here, vtop=vbottom=0)
     
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