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Buyoancy Question the day before the final!

  1. Dec 14, 2009 #1
    You place a beaker partly filled with water, into a sink. The beaker itself has a mass of 390grams and an interior volume of 500 cm^2. You now start to fill the sink with water and you f ind that if the beaker is less than half full, it floats but it is more than half full it sinks to the bottom.

    What is the density of the glass making up the beaker?

    The answer is 2786 kg/m^3

    How do you do this? argh i got my final tomorrow =S
    1. The problem statement, all variables and given/known data

    mass of beaker=0.39 kg
    density of water is 1000 kg/m^3
    interior volume of beaker is 500 cm^2
    2. Relevant equations

    Fb= density*g* Volume displaced

    3. The attempt at a solution
    i don't know how to do this AT ALL!
     
  2. jcsd
  3. Dec 14, 2009 #2

    Borek

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    Staff: Mentor

    What is the maximum water volume the beaker can displace?

    What is its mass (including water inside) if it starts to sink?
     
  4. Dec 14, 2009 #3
    max water the beaker can displace is
    mg= density*gravity*volume
    0.390 = 1000 x volume
    volume=3.9*10^-4

    Mass (including water) when it starts to sink...
    0.5*density*volume
    0.5*1000*5*10^-4=0.25 kg
     
  5. Dec 14, 2009 #4

    Borek

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    Staff: Mentor

    No. Think - if you put 500 mL beaker in water up to the rim, how much water was displaced?
     
  6. Dec 14, 2009 #5
    ummm 5x10^-4 m^3
     
  7. Dec 14, 2009 #6

    Borek

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    Staff: Mentor

    Yes. m3 is not the easiest unit to work with in this case, but it is correct (just inconvenient).

    What mass of water was displaced?

    If beaker starts to sink, what does it tell about its mass?
     
  8. Dec 14, 2009 #7
    is the mass of water displaced 0.5*density of water*volume of beacker..
    as the beaker starts to sink, it tells me that the mass will is increasing?
     
  9. Dec 14, 2009 #8

    Borek

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    Staff: Mentor

    Why 0.5?

    :bugeye:
     
  10. Dec 14, 2009 #9
    haha i meant as the beaker starts to sink, we know that the mass of the water inside the beaker is making it sink to the bottom.
    It is 0.5 because it's half the volume of the beaker =)
     
  11. Dec 14, 2009 #10
    V(outer voume of beaker)*g*1000 = v/2(v=inner volume)*g*1000 + m(m=mass of beaker)*g


    v=500cm^3 = 500*10^-6 m^3
    g=10m/s^2
    m=390gm=.39kg

    then density = mass of beaker / volume of beaker


    volume of beaker = V-v
     
  12. Dec 14, 2009 #11
    Thanks
     
  13. Dec 14, 2009 #12
    did u get it?
     
  14. Dec 14, 2009 #13
    ok ur welcome
     
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