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Conceptual Question about Buoyancy

  1. Mar 29, 2012 #1
    Say that I have two objects of the same mass and volume. I place one into freshwater and another in glycerine. In both cases, the objects are floating. Would the buoyant force from both of these fluids acting on the object be the same?

    From my understanding, buoyant force measures the weight of an object if it's floating. Since both of the objects are of the same weight and are afloat , does it make sense to think that the buoyant force would be the same in the two situations?

    Many thanks.
     
  2. jcsd
  3. Mar 29, 2012 #2

    Doc Al

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    Yes.

    Since the objects are floating, the buoyant force must equal the weight.
     
  4. Mar 29, 2012 #3

    Dale

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    Note that the waterline on the hull will not be the same as the glycerin line. I.e. They will displace different volumes of fluid to get the same buoyant force.
     
  5. Mar 29, 2012 #4

    A.T.

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    Now imagine the two identical objects are not floating, but hanging on a scale like this:

    waage_f1c.jpg

    Your water and glycerin are in buckets standing on a scale like this:

    scale.gif

    Both scales are initially balanced. Then you submerge the objects into the buckets, and they do not float, but submerge fully (without touching the walls).

    Does the balance of the scales change? If yes, how?
     
  6. Mar 30, 2012 #5
    Hmm...that's a really good question. I would think that the balance of the scales would not change. This is because the displaced fluid from the fully submerged object does not escape from the buckets and thus, the mass in both of buckets remain equal when the objects are added. This is just my thinking though. Is this correct?
     
  7. Mar 30, 2012 #6

    Doc Al

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    Consider the buoyant force on each submerged mass. Is it the same for each?
     
  8. Mar 30, 2012 #7

    A.T.

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    Try to figure out the upper scale first, where the objects hang, and consider what DocAl said.
     
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