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Need help finding spring constant

  1. Feb 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A 147g ball is dropped from a height of 56.5cm above a spring of neglibible mass. The ball compresses the spring to a maximum displacement of 4.45393cm. The acceleration is 9.8m/s^2. calculate the spring force constant k. Answer in units of N/m.

    2. Relevant equations

    Not sure


    3. The attempt at a solution

    I converted the mass from 147g to .147kg the height from 56.5cm to .565m and the displacement from 4.45393cm to .0445393m.

    Now im stuck and don't know what to do

    I tried mgh=.5kx^2 but didnt work
     
  2. jcsd
  3. Feb 27, 2008 #2

    Doc Al

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    Staff: Mentor

    What did you use for h? (You need the change in height, from initial position to final position.)
     
  4. Feb 27, 2008 #3
    what do you mean? i used the given height of .565m
     
  5. Feb 27, 2008 #4

    Doc Al

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    Staff: Mentor

    Hint: What's the change in gravitational PE? Hint 2: Measure gravitational PE from the lowest point.
     
  6. Feb 27, 2008 #5
    so use a zero height? or use .565+.0445393?
     
  7. Feb 27, 2008 #6

    Doc Al

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    That's the height you need to use in that formula. This is just a consequence of conservation of energy.
     
  8. Feb 27, 2008 #7
    Thanks you are a genius
     
  9. Feb 27, 2008 #8

    Doc Al

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    Staff: Mentor

    You are very perceptive. :rofl:
     
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