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Homework Help: Buoyant Force on a man

  1. Apr 24, 2017 #1
    1. The problem statement, all variables and given/known data
    A certain man has a mass of 85.0 kg and a density of 995 kg/m3 (exclusive of the air in his lungs).
    (a) Calculate his volume.
    .0854 m3
    (b) Find the buoyant force air exerts on him.
    (c) What is the ratio of the buoyant force to his weight?
    (buoyant force / weight)

    2. Relevant equations
    Fb=density (of substance) x Volume (of object) x g
    density of air=1.2 kg/m3

    3. The attempt at a solution

    Fb=(1.2 kg/m3) x (9.8) (.0854)

    and then for part c
    1.004/ (9.8 x 85=833)=.00121

    For some reason this is incorrect. Does anyone see why?
  2. jcsd
  3. Apr 24, 2017 #2


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    Incorrect density.

    Edit: Sorry I misread the question.
    Last edited: Apr 24, 2017
  4. Apr 24, 2017 #3


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    For the buoyant force of the displaced air, the air density should be used as the OP shows. How far off is the answer? Could it be just due to rounding of 1.225 to 1.2?
  5. Apr 24, 2017 #4
    Yes, I tried that as well and obtain 833N, which is also incorrect. I did this first, and then thought we need to use the density of the air.
  6. Apr 24, 2017 #5
    yes, I tried 1.225 and 1.2. Both are incorrect.
  7. Apr 24, 2017 #6


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    Just so we don't have to keep guessing, what is the answer given?
  8. Apr 24, 2017 #7
    I don't have access to the answer yet-it's a web assign HW question.
  9. Apr 24, 2017 #8


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    Staff: Mentor

    Try to make sure that all constants used have as at least as many significant figures as the given data. That includes your value for g.
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