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Homework Help: Calculating the spring constant involving energy

  1. Oct 2, 2011 #1
    1. The problem statement, all variables and given/known data
    A 142 g ball is dropped from a height of 62.2 cm above a spring of negligible mass. The ball compresses the spring to a maximum displacement of 4.35501 cm. acceleration due to gravity is 9.8. Calculate the the spring force constant K.


    2. Relevant equations

    Hooke's law: F=-kx and the potential energy of the spring is given by mg(h+x) because total displacement involves the compression distance x as well as the height of the ball

    3. The attempt at a solution
    So I got mg(h+x)=-kx,
    -mg(h+x)/x=k
    (-.142*9.8*(.622+.435501))/.435501=k but I keep getting the wrong answer. What am I doing wrong am I supposed to use PE=1/2kx^2 instead?
     
  2. jcsd
  3. Oct 2, 2011 #2

    Doc Al

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    Staff: Mentor

    You can't set an energy term equal to a force term.
    Yes.
     
  4. Oct 2, 2011 #3
    ok so you get -(mg(h+x)*2)/x^2=k,

    -(2*.142*9.8*(.622+.435501)/.435501^2=k
     
  5. Oct 2, 2011 #4

    Doc Al

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    Staff: Mentor

    Get rid of that minus sign. And be careful when converting cm to m: 4.35 cm = 0.0435 m.
     
  6. Oct 2, 2011 #5
    sweet thanks!!
     
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