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By how much is each post compressed?

  1. May 3, 2017 #1
    1. The problem statement, all variables and given/known data
    A large 30000 L aquarium is supported by four wood posts at the corners. Each post has a square 6.00 cm x 6.00 cm cross section and is 90.0 cm tall. Take Y=10^10 for wood

    2. Relevant equations
    ΔL = FL/AY
    F = ma
    ρ = m/V

    3. The attempt at a solution
    So the total area is 0.06 * 0.06 * 4 = 0.0144m2
    The length of each post is 0.9m
    The total mass of water is 30000kg (taking the density as 1kg/L)
    Meaning the weight is 30000*9.8 = 294000N
    And Y=1010

    Which gives:
    ΔL = (294000*0.9)/(0.0144*1010
    = 0.00184

    But this is apparently wrong. I have no idea what I did wrong
     
  2. jcsd
  3. May 3, 2017 #2

    gneill

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    Staff: Mentor

    Were they expecting the answer to be given in particular units?
     
  4. May 3, 2017 #3
    It has never mattered before as I can specify the units. So it shouldn't matter.
     
  5. May 3, 2017 #4

    gneill

    User Avatar

    Staff: Mentor

    Well, I can't spot any other issues. Your calculations look okay to me. Unless of course this is a salt-water aquarium?
     
  6. May 3, 2017 #5
    That was my second thought. In which case I get 0.0019 which was also incorrect. So yeah, no idea what the issue is maybe they do want different units so I may try that otherwise I don't know. I only get 5 attempts (this is mastering physics)
     
  7. May 3, 2017 #6
    Turns out every other question wants two significant figures except this one which wanted exactly 3 to be correct so yeah. 0.00184 was correct. Thanks for the assistance
     
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