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Potential Energy with springs

  1. May 13, 2015 #1
    1. The problem statement, all variables and given/known data
    A spring constant of ##3200 N/m## is initially streched until the elastic potential energy is ##1.44## J (##U=0## for no stretch). What is the change in elastic potential energy if the initial stretch is changed to (a) a stretch of ##2.0## cm, (b) a compression of ##2.0## cm, (c) a compression of ##4.0## cm.


    2. Relevant equations
    ##\Delta U=U_f-U_i##
    ##U=\frac{1}{2}kx^2##

    3. The attempt at a solution

    (a) ##\Delta U=U_f-U_i=\frac{1}{2}(3200)(0.02)^2-1.44=-0.8 J##

    (b) ##\Delta U=U_f-U_i=-\frac{1}{2}(3200)(0.02)^2-1.44=-2.08 J##

    (c) ##\Delta U=U_f-U_i=-\frac{1}{2}(3200)(0.04)^2-1.44=-4J##

    The first answer (a) is correct but the last 2 they get (b) ##-0.8J## and (c) ##1.1J## respectively. How?
     
  2. jcsd
  3. May 13, 2015 #2

    PhanthomJay

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    You should check your math.....
     
  4. May 13, 2015 #3
    If the spring is compressed, isn't potential energy for the spring negative?
     
  5. May 13, 2015 #4

    PhanthomJay

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    square a negative and you get a ????
     
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