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Two Questions on Archimedes's Principle

  1. Dec 11, 2006 #1
    1. The problem statement, all variables and given/known data
    A block of wood weighs 50.0 N when weighed in air. A sinker is attached to the block, and the weight of the wood-sinker combination is 200 N when the sinker alone is immersed in water. Finally, the wood-sinker combination is completely immersed and the weight is 140 N. What is the density of the block?



    2. Relevant equations
    (I'm not so sure how to write out equations on here, being my first post, so bear with me...)

    B=(rho)*V*g
    SumF_y=0
    (rho)=m/V
    And that's really all I can come up with...


    3. The attempt at a solution
    I haven't been able to get a good start on this problem, but I think it will have to do with the buoyancy force-weight of the system=0. I'm really not sure on this one...


    And the second question... I probably shouldn't expect anyone to waste this much time helping me, lol...

    1. The problem statement, all variables and given/known data
    A solid copper ball with a diameter of 3.00 m at sea level is placed at the bottom of the ocean, at a depth of 10.0 km. If the density of the seawater is 1030 kg/m^3, how much does the diameter of the ball decrease when it reaches bottom?


    2. Relevant equations
    (rho)=m/V
    V_sphere=[4(pi)r^3]/3
    P=P_o+(rho)gh


    3. The attempt at a solution
    P=1.01x10^5+(1030)(9.8)(1.0x10^4)

    My problem is, how do I relate the pressure, P, to the density and volume of the ball?



    I guess the majority of my problems are conceptual. I sometimes don't understand how to begin a problem, or which equations to use in order to find my variables. Does anyone have any good general tips for my situation? Thanks for looking, and I hope this isn't too much for someone to help me out with...
     
  2. jcsd
  3. Dec 11, 2006 #2

    OlderDan

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    Science Advisor
    Homework Helper

    Let's look at your problems one at a time. It is better not to post multiple problems in one thread becaue it can lead to confusion as people respond to one problem or another.

    For number one, use the information given in the problem to determine the buoyant force on the block when it is submerged. Once you know that you can find its volume, and since you know its true weight, you can find its density.
     
  4. Nov 15, 2009 #3
    Did you ever solve this? I have the exact same question and need some help.

    any ideas? I came up with a density for the block of wood of 5000 g/cm^3

    feel free to reply to <email address deleted>

     
    Last edited by a moderator: Nov 16, 2009
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