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Buoyant Force and Newtons

  1. 5.0 N

    0 vote(s)
    0.0%
  2. 3.5 N

    0 vote(s)
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  3. 8.5 N

    0 vote(s)
    0.0%
  4. 1.5 N

    1 vote(s)
    100.0%
  5. Other

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    0.0%
  1. Sep 26, 2007 #1
    1. The problem statement, all variables and given/known data
    An object weighing 5.0 Newtons displaces a certain volume of water when dropped into a large beaker of water. The weight of the displaced water is 3.5 Newtons. The object is free to sink or float. What would be the value of the bouyant force is another object twice the volume, but having the same weight, was droppen into the large beaker of water? The object is free to sink or float

    I believe the bouyant force is 3.5 Newtons, judging from Archimedes' Principle which states that "an object immersed in a fluid experiences a bouyant force equal to the weight of the water displaced." I'm not sure if this is correct, going by volume and not weight.
     
  2. jcsd
  3. Sep 26, 2007 #2

    Doc Al

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    The first thing to decide is whether it sinks or floats. Well?

    Does that one sink or float?
     
  4. Sep 26, 2007 #3
    They both sink.
     
  5. Sep 26, 2007 #4

    Doc Al

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    Explain your reasoning.
     
  6. Sep 26, 2007 #5
    Because an object will float if the bouyant force (3.5 N) is greater than the weight of the submerged object (5 N) - so in this case the object would sink.

    But does the bouyant force change if the volume is doubled?
     
  7. Sep 26, 2007 #6

    Doc Al

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    Good. This means that the entire volume is underwater.

    What matters is the volume of the object that is under water.

    Hint: If this second object were totally submerged, what would be the buoyant force on it?
     
  8. Sep 26, 2007 #7
  9. Sep 27, 2007 #8

    Doc Al

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    No. Recall that, per Archimedes' principle, the buoyant force equals the weight of the displaced fluid. The first object was totally submerged, thus it displaced an amount of water equal to 3.5 N. But the second object has twice the volume. So if that second object were totally submerged, it would displace twice the water. How much buoyant force would that equal?

    Does the second object float or sink?
     
  10. Sep 27, 2007 #9
    It would float- so 5 N?
     
  11. Sep 27, 2007 #10

    Doc Al

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    Right. If the 2nd object were totally submerged it would displace twice the water and thus have twice the buoyant force: 7 N. That's more than enough to support its weight, so it will float only partly submerged. Since you know it floats, its buoyant force must equal its weight = 5 N.
     
  12. Sep 27, 2007 #11
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