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Homework Help: Volume (Math check)

  1. Nov 7, 2006 #1
    A small sculpture made of brass (p = 8470 kg/m^3) is believed to have a secret central cavity. The weight of teh sculpture in air is 15.76N. When it is submerged in water, the weight is 13.86N. What is the volume of the secret compartment?

    Fb = W
    W = pgV

    15.76 = 8470 * 9.8 * V
    15.76 = 83006V
    1.89x10^-4 = V

    13.86 = 8470 * 9.8 * V
    13.86 = 83006V
    1.66x10^-4 = V

    1.89x10^-4 - 1.66x10^-4 = 2.3x10^-6

    This is wrong. Why?
  2. jcsd
  3. Nov 7, 2006 #2


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    Homework Helper

    Wrong calculation: 1.89x10^-4 - 1.66x10^-4 = 2.3x10^-5.
  4. Nov 7, 2006 #3


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    I don't understand what you've done - you appear to have calculated 2 volumes. The volume is constant.

    Remember, the mass (weight) difference in water tells you the mass (weight) of water displaced. So use this to find the volume of water displaced - then you know the volume of the whole brass thing.

    Calculate what a SOLID brass thing of this volume should weigh.

    Find the "missing" mass or weight then the volume.

    There might be a quicker way. ......
  5. Nov 7, 2006 #4


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    Ah - you worked out the whole volume then the volume of just brass?

    But did it work?
  6. Nov 14, 2006 #5
    What I did is find the volume of the brass in the air, then the volume in the water and found the difference between the two. This is not the correct answer though. What did I do wrong?
  7. Nov 15, 2006 #6
    OK, think I got it now:

    V= m/p

    Air = 15.76 / 8470
    = 1.8 x 10^-3

    Water = 13.86 / 8470
    = 1.6x10^-3

    Difference of the two is 2x10^-4, which happens to be one of the answers.
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