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Homework Help: Elevator Cable Breaks

  1. Feb 3, 2010 #1
    1. The problem statement, all variables and given/known data

    An elevator cable breaks when a 798 kg elevator is 28.9 m above a huge spring (k = 8.00 multiplied by 104 N/m) at the bottom of the shaft.

    (a) Calculate the work done by gravity on the elevator before it hits the spring. I got 226009 which is right

    (b) Calculate the speed of the elevator just before striking the spring. I got 23.8 which is right

    (c) Calculate the amount the spring compresses (note that work is done by both the spring and gravity in this part).

    I need help with C!

    2. Relevant equations

    PEgr = mgh
    PEel = 1/2kx^2

    3. The attempt at a solution

    I dont have my work in front of me right now but I got 2.28 and 2.38 and they were each wrong. I'll edit this later if i find my work
  2. jcsd
  3. Feb 3, 2010 #2
    I remember now... I did it quadratically.

    My final set up was

    4.00e4 x^2 + 7828 x - 226009
  4. Feb 3, 2010 #3


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    PEel = 1/2kx^2
    It should be
    mg(h+x) = 1/2kx^2
  5. Feb 3, 2010 #4
    mg(h+x) = 1/2kx^2

    so would it be 7820(28.9+x) = 1/2(8*10^4)(x^2)

    then... 226009 + 7820x = 40000x^2

    or... -40000x^2 + 7820x + 226009 = 0

    then would x be 2.47???

    I'm only clarifying because I have one chance left to get it right
  6. Feb 3, 2010 #5


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    Your answer is correct.
  7. Feb 3, 2010 #6
    thank you so much
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