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Density, buoyancy, and volume in static fluids

  1. Nov 14, 2014 #1
    1. The problem statement, all variables and given/known data
    A rock is suspended by a light string. When the rock is in air, the tension in the string is 35.7N . When the rock is totally immersed in water, the tension is 25.9N . When the rock is totally immersed in an unknown liquid, the tension is 19.6N .

    2. Relevant equations
    Density = m/v
    buoyant force (b.f.)= density(of fluid) * gravity * Volume(of the part that is immersed in the fluid)

    3. The attempt at a solution
    Since we're given the tension (T) in the air, we can find the mass of the rock because T = mg in that case. Then after this I'm not sure where to go with it because since I don't know the volume of the rock I can't use the (b.f.) equation to help me at all. What am I missing?

    Thanks
     
  2. jcsd
  3. Nov 14, 2014 #2

    Bystander

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    "When the rock is totally immersed in water ...."
     
  4. Nov 14, 2014 #3
    I don't see how that helps me though. If I try using that info I come up with the equation:

    ρliquid * gravity * Vrock = 35.7

    Still have 2 unknowns...
     
    Last edited: Nov 14, 2014
  5. Nov 14, 2014 #4

    haruspex

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    Please explain how you obtain that equation from the relevant equations you posted.
     
  6. Nov 14, 2014 #5
    Sorry I meant to put 19.6 instead of 35.7. So the equation should actually read:

    ρliquid * gravity * Vrock = 19.6
     
  7. Nov 14, 2014 #6

    haruspex

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    No better. Draw a free body diagram of the rock suspended immersed in a fluid. What forces act on it?
     
  8. Nov 14, 2014 #7
    Rock in water/unknown fluid:
    Tension & buoyancy pointing up, of force gravity pointing down.

    ∑forces : Tension + B.F = mg
     
  9. Nov 14, 2014 #8
    If the rock weighs 35.7N, and, when it is immersed in water, the tension in the string is 25.9N, what is the upward force exerted on the rock by the surrounding water?

    Chet
     
  10. Nov 14, 2014 #9
    That would be 35.7-25.9=9.8?
     
  11. Nov 14, 2014 #10

    haruspex

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    Yes. Now apply that to the other liquid.
     
  12. Nov 14, 2014 #11
    From Archemides principle, what volume of water would have to be displaced to produce the 9.8 N upward force? How does that relate to the volume of the rock?

    Chet
     
  13. Nov 14, 2014 #12
    Ooh I get it!

    I found the volume of the rock to be .001
    and I plugged in that value into the B.F equation for the unknown liquid
    then I set that equal to the weight of the rock - the tension and found the density.

    Thank you very much!
     
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