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Weird buoyancy question

  1. Jan 18, 2008 #1
    There is a container full of water with a hole in the bottom. The hole is sealed and fits water-tight around an enclosed cylinder (like a straw sealed at both ends). The cylinder still moves easily vertically and it is inserted in such a way that it protrudes out both the top and bottom of the water. Ideally the theoretical bouyancy of the part in the water overcomes the protruding ends weight. Would this cylinder float up through the hole??

    Please help
     
  2. jcsd
  3. Jan 18, 2008 #2

    Doc Al

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    Staff: Mentor

    Ask yourself: What causes buoyancy? What force does the water exert on the cylinder?
     
  4. Jan 21, 2008 #3
    I have.... a lot. That's why I came to the forum. I know that the definition of buoyancy has to do with displacement weighing more than the body itself but then I had to start thinking of why this happens. This seems out of my spectrum, hence me coming to the forum.
     
  5. Jan 21, 2008 #4

    Doc Al

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    My question is simpler: What causes the buoyant force (if it exists)? Something must be exerting the force. What is it? This may sound like a silly question, but answer it anyway. (This will help you better understand buoyant force.)

    Archimedes's principle states that the buoyant force on a submerged object equals the weight of the displaced fluid. But there are important caveats.
     
  6. Jan 21, 2008 #5
    I found out there would not be a bouyant force. I wish school would not "define" buoyancy with its formula. I found your reply to someone asking about a perfect seal cone on the bottom of a pool, which applies to my question.

    EDIT: I thought of a new question now.... why do bubbles "stick" to the sides of my freshly poured soda? Is cohesion involved? Cohesion of the water and glass right above the bubble maybe?
     
    Last edited: Jan 21, 2008
  7. Jan 21, 2008 #6

    Doc Al

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    Right. The "buoyant" force is just the result of the fluid pressure exerting a net force on an object. (The fact that pressure changes with height makes a net force possible.) But the entire submerged surface of the object must be in contact with the fluid in order to apply Archimedes's principle.

    In your example, as you realize, the water is only in contact with the sides of the cylinder, not the bottom. The net force exerted by the water is zero.

    And I agree that many introductory treatments treat Archimedes's principle as if it were more fundamental than it is.
     
  8. Jan 21, 2008 #7
    Thank you, I have been enlightened. I like this "instant message" style of discussion lol.
     
  9. Jan 21, 2008 #8

    DaveC426913

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    Gold Member

    Ohhh...

    You're talking about this:

    Code (Text):

       _
      | |
      | |
     _| |_
    | | | |
    | | | |
    | | | |
    |_| |_|
      | |
      |_|

     
    I thought you were talking about this:

    Code (Text):
       _
      | |
      | |
     _| |_
    | | | |
    | | | |
    | |_| |
    |_____|
    ...which would cause problems with vacuum pressure.
     
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