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Homework Help: Physics fluids - buoyant principle help

  1. Jan 29, 2004 #1
    A problem states that a man with a mass of 55 kg floats with 90% of his body under water.

    I jus need to find the weight of his body when submerged in water...i'm was thinking 90% of 55 kg, but i wasn't sure if that was correct....
  2. jcsd
  3. Jan 29, 2004 #2


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    Apparent weight is the weight minus the buoyant force: W-BF

    Buoyant force is equal to the weight of water displaced. The BF is equal to the weight of the water displaced. If 90% of the dude's volume is displacing water, that force caused by the displacement is proportional to 90% of his volume. Furthermore the guy is staying, put he's not going up or down...think about what this means, you don't need to know the volume or even the weight for this problem.
    Last edited by a moderator: Jan 29, 2004
  4. Jan 30, 2004 #3

    Chi Meson

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    This looks like a trick question.

    "weight" is an ambiguous term: it could mean the "force due to gravity," in which case the man's weight is mg. "weight" could mean (as was stated) the apparent weight which is mg-BF. IF this is the case, then it is zero because the man is floating.

    If you are floating with some of your body above the surface, then the bouyant force is equal to the density of water times "g" times the "submerged volume" of the body; the gravitational force is still the entire mass of the whole body times "g." When floating, these forces are in equilibrium.
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