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I am having some trouble with conservation of energy

  1. Mar 30, 2008 #1
    1. The problem statement, all variables and given/known data
    [​IMG]


    2. Relevant equations
    [tex]K_{1}[/tex]+[tex]U_{1}[/tex]+[tex]W_{nc}[/tex]=[tex]K_{2}[/tex]+[tex]U_{2}[/tex]


    3. The attempt at a solution
    Using the above equation

    1/2m[tex]v^{2}[/tex] + 0 + xmg[tex]\mu[/tex] = 0 + 1/2k[tex]x^{2}[/tex]

    On the left side
    When it hits the spring there is 0 potential because it has not yet compress the spring and it still remains on a horizontal surface. It has a kinetic energy and kinetic friction work
    On the right side
    I have 0 kinetic because it just finished compressing the spring and about to decompress so instead I get maximum potential energy.

    So solving for K i get

    [tex]\frac{mv^{2}+xmg\mu}{x^{2}}[/tex]

    But according to the answer it does not depend on the variable X (distance compressed by spring)
    So I'm not entirely sure I know where I made my mistake
     
    Last edited: Mar 30, 2008
  2. jcsd
  3. Mar 31, 2008 #2

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    Aside from your algebra error and your incorrect assumption that the work done by friction is positive, you have neglected to incorporate the given condition that the object has no speed when the spring returns to its uncompressed length (that is, its speed is zero both when the spring is fully compressed, and when it returns to its uncompressed state). That will give you the info you need to eliminate the 'x' term. Remember that the work done by friction is negative.
     
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