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Homework Help: Force and Buoyancy

  1. Apr 15, 2005 #1
    Can anyone help me out. I don't know of any equation that i can us eto solve this A force 1000N is required to raise a concrete block to the surface of a fresh wate rlake. what is the force required to lift it out of the lake. I know that i need to figure out what the weight outside of wate ris, but i don't know how. no i don't know the volume the 1000N is the only info given. density is 2320 kg/m3.
     
    Last edited: Apr 15, 2005
  2. jcsd
  3. Apr 15, 2005 #2

    FredGarvin

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    Science Advisor

    If you draw a free body diagram of the block under water you should have three forces acting on it: The block's weight, the buoyancy force and the 1000N applied force. This should tell you that the force required to lift the block in air will equal the buoyant force + 1000N.

    Do you know the volume of the block?
     
  4. Apr 15, 2005 #3

    Doc Al

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    Staff: Mentor

    But are you given the density of concrete?
     
  5. Apr 16, 2005 #4
    The Force you need to raise the block with the volume [tex]V[/tex] to the surface is
    [tex]F_1=gV(\rho_{concrete}-\rho_{water})[/tex]
    the force to raise the block futher is

    [tex]F_2=gV\rho_{concrete}[/tex]

    Then [tex]\frac{F_2}{F_1}=[/tex]
     
    Last edited: Apr 16, 2005
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