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Pressure 2 m deep in a tank full of water

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  1. Jan 21, 2016 #1
    1. The problem statement, all variables and given/known data
    In a tank full of water, the pressure on a surface 2 meters below the water level is
    1.5 kPa. What is the pressure on a surface 6 meters below the water level?

    2. Relevant equations
    Isn't this impossible? The equation given is P=hdg. (height of water column x density x 9.8m/s/s) and the density of water is 1.

    3. The attempt at a solution
    So by that equation, the pressure 2 meters below would be P=2m x 1 x 9.8m/s/s = 19.6 kPa
    and the pressure 6 meters below the surface would be P= 6m x 1 x 9.8m/s/s = 58.8 kPa

    Am I misunderstanding something here?

    Thank you in advance!
     
  2. jcsd
  3. Jan 21, 2016 #2

    mfb

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    This equation gives zero pressure at a height of 0. Possible, but it is a special case that does not have to be true. There is a more general formula. Alternatively, consider pressure differences only.
     
  4. Jan 21, 2016 #3

    haruspex

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    The planet is not specified. But then, neither is its atmospheric pressure, making the whole indeterminate.
     
  5. Jan 22, 2016 #4

    mfb

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    I guess we can assume this happens at the surface of Earth. Atmospheric pressure does not influence the result (but we have to assume that 1.5 kPa are pressure relative to atmospheric pressure otherwise the described system cannot exist).
     
  6. Jan 22, 2016 #5

    haruspex

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    But Spiffy's point is that the set up described is not possible under those conditions. The excess pressure would be more like 20kPa at 2m. That's why I brought up the possibility of a different context.
     
  7. Jan 22, 2016 #6

    mfb

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    Excess pressure relative to the highest point in the tank, which can be below atmospheric pressure.
     
  8. Jan 22, 2016 #7

    haruspex

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    Sure, but it can't be negative. Even if the tank is under a vacuum the pressure at 2m will be far more than the question states.
     
  9. Jan 23, 2016 #8

    mfb

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    Sometimes pressure is given relative to atmospheric pressure. I suggest to wait for SpiffyPhysics.

    @SpiffyPhysics: Is this the full and exact problem statement?
     
  10. Jan 23, 2016 #9

    haruspex

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    Let the absolute pressure at the surface be P. On Earth, the pressure at 2m depth of water will be about P+20kPa, yes? What value of P is going to give a resulting pressure of a mere 1.5kPa?

    The only interpretation that makes sense to me is a) this is not on Earth, and b) the pressures are relative to atmospheric, whatever that is.
     
  11. Jan 23, 2016 #10

    ehild

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    Or the fluid is not water.
     
  12. Jan 23, 2016 #11

    TSny

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    The pressures are gauge pressures and the tank is in an elevator.:smile:
     
  13. Jan 23, 2016 #12

    haruspex

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    It says water.
     
  14. Jan 23, 2016 #13

    haruspex

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    An accelerating elevator... Albert E says that might as well be a different planet.
     
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