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Homework Help: 9 logs, how many people?

  1. Feb 27, 2009 #1
    A raft is made of 9 logs lashed together. Each is 29.1 cm in diameter and has a length of 5.7 m. How many people can the raft hold before they start getting their feet wet. Assume the average person has a mass of 69.3 kg and that the density of water and wood is 1000 kg/m3 and 600 kg/m3, respectively

    [okay, what i did was i found the volume of one log.
    V = (pi)(r^2)H
    r is used 0.1455 (29.1 converted to m then divided by 2 to get radius)
    h i used 5.7m , then times it by 9.
    (i got an answer of 3.411875)

    then i found FB. FB = pwaterVg (1000)(3.41188)(9.81)

    since i knew the density of wood, and the volume, i found the mass
    m = pwoodV
    got an answer of 2047.125

    then i used this equation FB = [ x*mass of one person + mass of 9 logs] g

    when im done, i get an answer of 46 people. its wrong. can someone show me where im going wrong!?
  2. jcsd
  3. Feb 27, 2009 #2


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    Homework Helper

    From your numbers you have 3411 - 2047 = 1364 and divide by 69.3 gives a different number.
  4. Feb 27, 2009 #3
    ahh! ok, i see now, !
    makes sense, i didn't need to multiply it by 9.81!
    Thanks :)
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