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Homework Help: Quick question on fluids and buoyancy

  1. Dec 15, 2004 #1
    Hi, i have a quick question regarding the above subject.


    F(b) = (rho) (V) (9.8)

    and F(b) = F(g)

    and F(g) = (m) (9.8)

    then wouldnt F(b) = (m) (9.8)

  2. jcsd
  3. Dec 15, 2004 #2


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    A = B
    B = C

    Therefore, A = C

    I don't see why there should be any problem...
  4. Dec 15, 2004 #3
    Then why do we need to use the formula:

    F(b) = (rho) (V) (9.8) to find the buoyancy force when we could just multiply its mass by 9.8 ??
  5. Dec 15, 2004 #4


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    Frog knows.

    Really, what is the context of the question? If the object is floating - ie. weight = bouyancy, then obviously compute the weight and you get the bouyancy. Careful with V though - the equation will give you the submerged volume of the object, not the total volume of the object.
  6. Dec 15, 2004 #5
    the question is:

    a submarine with volume(total) 4.67x10^3 and mass of 4.39x10^6 is floating at the surface of sea water, of density 1.03x10^3

    a) what is the buoyant force on the submarine?


    In this case, we do not have enough information to use the equation F(b)=(rho)(V)(9.8)

    since m = (rho)(v) - thus, (rho)=m/v

    sub it in and you get

    F(b) = (m/v)(v)(9.8)
    F(b)= (m)(9.8)
  7. Dec 15, 2004 #6
    another quick question when we plugin values for density for water do we use 1.0 or 1000
  8. Dec 15, 2004 #7


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    Examine the units that you were given. The density of water is 1000 kg/m^3 and 1.0 kg/dm^3 or 1.0 g/cm^3.

    If you keep the units consistent, you should be fine.
  9. Dec 15, 2004 #8
    What are we trying to match the units of density with?
  10. Dec 15, 2004 #9


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    All your other units.
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