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Conceptual problems with pressure, please help me to clarify

  1. Apr 10, 2009 #1
    Why is air pressure always factored in when calculating pressure at a given depth of water?
    Why is the force from air pushing down on the surface of water the same at 1m as it is at 100m?
    there is no air underwater, so why do we still need this value when doing pressure-depth calculations?
     
  2. jcsd
  3. Apr 10, 2009 #2

    mathman

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    Total pressure at any depth in the water is sum of air pressure and water pressure. The air pressure at the surface, independent of depth, has to be added in to get the total.
    For a simple analogy, put a ten pound weight on a scale - it will read ten pounds. Put another ten pound weight on top of the first and the scale will read twenty pounds.
     
  4. Apr 10, 2009 #3

    arildno

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    The water must be able to sustain the weight of the air above it.

    Thus, mathman's analogy is hardly just an analogy, rather, it is more of a precise description.

    As I am sure he knows full well.
     
  5. Apr 10, 2009 #4
    this was explained really well, thank you. just quick follow-up.. Is guage pressure simply pressure minus the air pressure? (101325Pa)?
     
  6. Apr 10, 2009 #5

    russ_watters

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    Yes.

    Btw, air pressure is not the same at 1m as at 100m. Pilots use barometers (air pressure sensors) to measure their altitude.
     
  7. Apr 11, 2009 #6
    I'm sure (!? uh -ohh better to stop typing now) that he meant 1 meter or 100 meters depth.
     
  8. Apr 12, 2009 #7
    I think he meant something like that, yes.

    I quote:
    This question makes no sense. There is no surface at 1m (depth?) or 100m. I'm guessing that you meant why you always have to add up the air pressure at the surface of the water whether we're talking about (total) pressures at 1m depth or 100m depth. I think the OP understands the answer already, but just to be sure. If you put three hamburgers on top of each other and you squash the top one, the bottom one will also get squashed, and not only the first (and second). :)

    I'm hungry now.
     
  9. Apr 12, 2009 #8

    russ_watters

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    Perhaps, but does it really matter...?

    If you drop a closed 100m pipe into the water, you can still use a barometric altimeter in it.
     
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