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Homework Help: Spring Problem

  1. Dec 19, 2007 #1
    The first part of this problem says that a 8.0kg stone rests on top of a spring, and compresses it by 10.0cm. It then asks what the spring constant is. This is the work I did (assuming g = 10m/s^2, not 9.8m/s^2):

    (8.0kg)(10m/s^2) = 1/2k(10cm)^2; 80N = 50cm; k = 1.6

    But, the textbook says the answer is .8. I don't understand why it's half my answer. Any help would be greatly appreciated.
  2. jcsd
  3. Dec 19, 2007 #2


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    Homework Helper

    On the left hand side, you have the gravitational force,
    [tex]F_g = m g[/tex]
    On the left hand side, you have the spring potential energy,
    [tex]W_s = \frac12 k x^2[/tex]
    Try taking the force exerted by the spring as well
    [tex]F_s = k x[/tex]

    Actually, you could have noticed this mistake by looking at the units... you wrote down 80N = 50cm. Even if you meant
    80 N = 50 cm^2 * k (N/cm)
    you would have equated N to N cm.
    Try to train yourself in spotting these mistakes: unless you know how to convert Newtons into Newtons times centimeters, an alarm bell should go off automatically.
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