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Path of least resistance

  1. Nov 21, 2006 #1
    If I put a 6 inch 40 foot pipe down into the ocean or large body of water, straight up and down, the bottom of the pipe is closed, top is opened above into the air, will the air pressure that has migrated down through the pipe be at a constant pressure as long as the pipe is strong enough to hold its shape?

    novice......
     
  2. jcsd
  3. Nov 21, 2006 #2

    berkeman

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    Staff: Mentor

    Sounds like a homework problem, so I moved the thread to the homework forums.

    Welcome to PF, GammaRayBurst. In your problem statement, would there be anything different between the situation you describe, and the same situation with no water? Like if you put the pipe in a lake with the open top above the surface, and then drained the lake. If the pipe is rigid....
     
  4. Nov 21, 2006 #3
    That works for me, thanks.
    Do you have any insight on under water implosions ?, like if a two liter pop with air inside was weighted to go down deep in the water, about how many feet would it take to implode?
    By the way its not homework, been out of school since 79, just a home tinkerer.
    thanks, don
     
  5. Nov 21, 2006 #4

    berkeman

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    Hi Don. In your original post (OP), you refered to a rigid pipe, so implosion wasn't part of the problem statement. In the case of the rigid pipe, then the water doesn't factor in at all. The air pressure distribution is the same as free air at that altitude throughout the pipe (a little higher at the bottom of the pipe, due to the slight weight of the air above it in the pipe).

    But for a plastic soda torpedo tube that is weighted and sealed and dropped in a lake, it will sink relatively intact until a depth where the water pressure exceeds the crush strength of the container. Keep in mind that spheres and cylinders are pretty strong against outside pressure. They are less strong if they have a dent somewhere in them -- that keeps the forces from being even (isotropic) around the container, and helps them crush at a lower overall pressure/depth.

    Does that make sense?
     
  6. Nov 21, 2006 #5
    I meant a two liter bottle filled with air.
    would it take about twenty feet or more before it imploded?
     
  7. Nov 21, 2006 #6

    berkeman

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    That sounds like a good guess, but I don't know for sure. I see an experiment in your Thanksgiving weekend plans.....

    And you understand that it's the straight sides of the cylinder shape that make the bottle crush at such a shallow depth, right?. A perfect plastic sphere would last much deeper.


    EDIT -- Changed my typo cylinder --> sphere in my last sentence.
     
  8. Nov 22, 2006 #7
    I want the tube to be solid, but the bottle would be like a collapsible bladder.


    You said: The air pressure distribution is the same as free air at that altitude throughout the pipe (a little higher at the bottom of the pipe, due to the slight weight of the air above it in the pipe.


    so the density of the air would be heavier at the bottom of the pipe?
     
    Last edited: Nov 22, 2006
  9. Nov 22, 2006 #8

    berkeman

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    Yes, the same as it is for normal atmospheric situations. The density of air is higher at sea level than it is at 10,000 meters up, right?
     
  10. Nov 22, 2006 #9
    Thanks again, I guess I should have already known this, but I seem to be having alot of brain farts on this project, but so far so good.
    Appreciate the help!
     
  11. Dec 27, 2006 #10
    Can one induce an implosion earlier in the bottle before it reached its crush depth by a simple low power means?
    perhaps a current from a stungun that is wired into the bottle.
    PS, just thinking in theory.

    Also, can anyone remind me of the liquid that when you apply an electric current, it becomes a solid?
     
    Last edited: Dec 27, 2006
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