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Work Done by Spring

  1. Nov 2, 2011 #1
    1. The problem statement, all variables and given/known data

    A spring with a spring constant of 10 N/cm has a cage attached to one end (Fig. 7-53). Taking rightward as the positive direction, (a) how much work does the spring force do on the cage when the spring is stretched from its relaxed length by 7.6 mm? (b) How much additional work is done by the spring force when the spring is stretched by an additional 7.6 mm?

    DP8KM.jpg
    2. Relevant equations

    W= -(½)*k*(d)^2


    3. The attempt at a solution
    First, I changed the units.
    d=.0076m
    k=1000 N/m

    For

    a) W = ½*1000*x0.0076² = 0.02888 J


    b) W = ½*1000*(0.0152² - 0.0076²) = 0.08664 J

    I know that I did the right work but I am still wrong. Does the answer have to be negative? But the problem states that the right is positive. I can't find my mistake.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 2, 2011 #2
    any help?
     
  4. Nov 2, 2011 #3

    PeterO

    User Avatar
    Homework Helper

    While stretching the spring to the right, you have done a positive amount of work. All that time the spring has been pulling to the left, so the spring force has been doing negative work.
     
  5. Nov 2, 2011 #4
    Thanks alot for helping making this clear for me.
     
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