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Homework Help: Bouyant Problems

  1. Jan 10, 2010 #1
    1. The problem statement, all variables and given/known data
    A beaker of mass 1.4 kg containing 2.3 kg of water rests on a scale. A 2.8 kg block of a metallic alloy of density 4700 kg/m3 is sus- pended from a spring scale and is submerged in the water of density 1000 kg/m3 as shown in the figure.

    (a) What does the hanging scale read? The
    acceleration of gravity is 9.8 m/s2 .
    Answer in units of N.
    (b) What does the lower scale read?

    2. Relevant equations
    P = Po + pgh
    Fb = Fdown - F up



    3. The attempt at a solution

    idk how to start on this one, i noe theres a Fup which P*A but the area is not given. =[ and neither is the heights and stuff so how am i sppose to find the bouyant force? and also for part ( b) is it simply just the mass of the beaker+ mass of water * g? since the fluid displaces is the same as the volumn of the mass
     
  2. jcsd
  3. Jan 10, 2010 #2

    Doc Al

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    Staff: Mentor

    Hint: Use Archimedes' principle to calculate the buoyant force on the block.
     
  4. Jan 10, 2010 #3
    The apparent weight " loss " of the object immersed in the fluid = the weight of the liquid displaced.

    so in this case, the Fb would be weight of object? and the reading on hanging scale would be Fg - Fb?
     
  5. Jan 10, 2010 #4

    Doc Al

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    Staff: Mentor

    The buoyant force equals the weight of the displaced fluid.

    The buoyant force equals the weight of the displaced fluid, not the weight of the object.
    Right!
     
  6. Jan 10, 2010 #5
    hmm so how exactly do i find the displaced fluid? theres no dimensions for me to find the volumn of the object to find the displaced fluid
     
  7. Jan 10, 2010 #6

    Doc Al

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    Staff: Mentor

    You have the object's mass and density. Use that information.
     
  8. Jan 10, 2010 #7
    but the formula is Fb = p(fluid)gv(submerged) w/o the volume idk how else to solve it.
     
  9. Jan 10, 2010 #8

    Doc Al

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    Staff: Mentor

    You have all the information needed to find the volume of the object and thus the displaced fluid.
     
  10. Jan 10, 2010 #9
    wowww its a simple D = m/v formula -_-'' thnx!
     
  11. Jan 10, 2010 #10
    A frog in a hemispherical pod finds that he
    just floats without sinking in a fluid with a
    density of 1.27 g/cm3.

    If the pod has a radius of 5.4 cm and negli-
    gible mass, what is the mass of the frog?
    Answer in units of kg.


    Alright so for this one i converted the density to kg/m3 first so it would be .00127m/kg3.
    i also converted the radius which turned out to be .0054m. now i tried to find the volume by using the volume of the sphere and divide it by half. it turns out to be some VERY VERY small decimal and the answer was wrong.
     
  12. Jan 10, 2010 #11

    Doc Al

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    Staff: Mentor

    Redo that conversion--you've got it backwards.
    Redo that one as well.
     
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