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Water displacment question.

  1. Jul 5, 2009 #1
    well today we were in the pool together and i had just finished a drink that id had(bottle of pepsi) and somehow we got talking about displacement, see what i want to know is how much force is that 600ml bottle is creating when its under then water(remembering that its empty)

    i dont know anything about physics so nice simple terms would be nice but dont worry to much cause im sure i can work it out

    any help much appreciated

    Alex
     
  2. jcsd
  3. Jul 5, 2009 #2

    Doc Al

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    The upward buoyant force that the water exerts on the bottle is equal to the weight of the displaced water. (That's Archimedes's principle.)
     
  4. Jul 5, 2009 #3
    ok so because its about 600 cubic centimetres it should be exerting 600 grams or something like that.
     
  5. Jul 5, 2009 #4

    Doc Al

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    Sounds right.
     
  6. Jul 5, 2009 #5
    but hang on, what if the bottle was filled with sand, it would still displace the same amount of water but of course it would sink. how does that work?
     
  7. Jul 5, 2009 #6

    Doc Al

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    The buoyant force that the water exerts on the bottle will be the same, but that's not the only force acting on the bottle. You must also consider the weight of the bottle (and contents), which acts downward. When filled with sand, the weight will be greater than the buoyant force and the bottle will sink.
     
  8. Jul 5, 2009 #7
    ok i get it. thanks you've helped me a lot :)
     
  9. Jul 5, 2009 #8
    Anytime the density of the contents is greater than the density of water, the bottle will sink...but if weighed underwater it weighs less than on land by the weight of water displaced.
     
  10. Jul 5, 2009 #9

    HallsofIvy

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    Well, no. 600g dynes, the force of gravity on a mass of 600 grams.
     
  11. Jul 5, 2009 #10

    Doc Al

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    600 grams weighs about 5.89 Newtons or 5.89 x 105 dynes.
     
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