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Homework Help: Buoyant force

  1. Oct 9, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    density = mass/volume
    FB = rhoo vog for a floating object
    F = mg

    3. The attempt at a solution

    Buoyant force = force of gravity due to the board
    rhoo vo = m

    vo = area*height submerged
    vo = 2.4*.05
    vo = 0.12 m^3

    volume of the board = thickness*area
    = 0.15*2.4
    = 0.36 m^3

    To find the density of the board, I need to know the mass of the board. I tried using the buoyant force of a floating object, but I need the density of the object.

    Any help?
  2. jcsd
  3. Oct 9, 2012 #2
    For a body to be afloat, the mass of water displaced by its submerged part must be equal to the mass of the entire body. Which you have derived in the form rhoo vo = m. What else do you need to compute the density?
  4. Oct 9, 2012 #3
    I need the mass of the object. How do I find that?

    Is rhoo equal to the density of water?
  5. Oct 9, 2012 #4
    I think I may understand where you are confused. You can calculate how much mass is displaced, but you need to consider the full geometry of the board to find its average density.
    Last edited: Oct 9, 2012
  6. Oct 9, 2012 #5
    What else could that be? See #2 above.
  7. Oct 9, 2012 #6
    You should always start with intuition. Would the plank be floating if its density were equal to that of water?

    Does styrofoam float on water? Is it more or less dense than water?
  8. Oct 9, 2012 #7
    So the mass of the board= density of water*volume of water displaced
    Because the object is floating, the volume of the water displaced is equal to the volume of the submerged part.
    so volume of water displaced = vo = 0.12 m^3

    = 1000*0.12
    =120 kg

    density of boat = m/v

    = 333.3 kg/m^3

    Is my thinking correct?
  9. Oct 9, 2012 #8
    Does the solution seem reasonable? How could you check?

    Here's a hint. :-)
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