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Homework Help: Spring force and work

  1. Oct 27, 2007 #1
    1. The problem statement, all variables and given/known data

    Consider a spring with spring constant k = 8.42 N/m with an unstretched length of 0.250 m. How much work do you do on the spring in stretching it from a length of L1 = 0.450 m to a length of L2 = 0.559 m?

    2. Relevant equations

    F= kx
    W=(1/2) kx^2

    3. The attempt at a solution

    since work is area under the curve of x vs. F graph, I did:
    W=(1/2)(8.42)(0.559-0.450)^2 = 0.0500 J (wrong answer)
    wrong maybe because this works only when u start at the unstretched length?

    So I also tried W= (1/2) k xf^2 - (1/2) k xi^2 [where xf = 0.559 and xi=0.450)
    =0.464J (still wrong)

    What am i doing wrong?
  2. jcsd
  3. Oct 27, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    How do you know it is wrong? Do you have the answer, or are you inputting it in some automated homework system? Because I think you did the right thing (that is, one of your attempts is right) but you didn't round correctly.

    The correct attempt is the last one (tell me if you know about integration, then I can tell you why). But also your first remark, about the area under the curve, you are right. Try plotting the curve, marking the area you want to get and then try to see what you did wrong (in particular, also try marking the area you actually calculated in the first attempt).
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