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Fluid Dynamics: Pressure / Opening a hatch underwater

  1. Apr 29, 2007 #1
    Let's say that you're deep under the surface of the water in a submarine type thing.

    There is a hatch on the top.

    Let's say that the absolute pressure on the top of the hatch was calculated to be 2 atm.

    Inside the vessel, the pressure is only 1 atm.

    Someone is inside the vessel and the force needed to open the hatch must be calculated.

    Do you need to take into account the pressure inside the vessel?

    P = F/A

    Do you plug in 2 atm, or 1 atm?

    My physics teacher said that you only look at the pressure on the outside, but he does not remember why.
  2. jcsd
  3. Apr 29, 2007 #2


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    Gold Member

    Then why is he teaching physics? :confused:
    I know pretty much nothing about anything, but I do know that the force required to open the hatch is based upon the ratio of interior/exterior pressure.
  4. Apr 30, 2007 #3
    Not the ratio. The difference: F= (Po-Pi)S
  5. Apr 30, 2007 #4


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    Gold Member

    Good catch. I have to start being more accurate in my terminology. After a few beers I get sloppy.
  6. Mar 29, 2008 #5
    Wouldn't you take the difference in pressure? You have 2 atm pushing in and 1 atm pushing out, requiring you to push through 1 atm to open the hatch, right?
  7. Mar 29, 2008 #6

    Doc Al

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    Staff: Mentor

    Realize that you are responding to a thread that's been dormant for almost a year. In any case, the point about needing the difference in pressure was made by lpfr in post #3.
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