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Compressed springs

  1. Feb 1, 2015 #1
    1. The problem statement, all variables and given/known data
    A man compresses a 30 cm spring against a wall. If he applies a 100 N force and the wall is rigid, by how much does he shorten the spring? (k = 1000 N/m).

    2. Relevant equations
    Fs=-kx

    3. The attempt at a solution
    Drew a FBD and labeled applied force of 100 N and calculated the force of the spring resisting the applied force = 30N. At this point, I do not know where to go with this problem.
     
  2. jcsd
  3. Feb 1, 2015 #2

    haruspex

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    If the force applied is 100N, how can it be that the reaction from the spring is only 30N?
     
  4. Feb 1, 2015 #3
    Ok. I think I figured this out I just need my work checked please. So the formula is F = -kx. So 100 = 1000(30-x) and x = 30 cm. How does this look?
     
  5. Feb 1, 2015 #4

    haruspex

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    I don't think you mean x=30. Also, be careful with units. Put all distances in metres.
     
  6. Feb 1, 2015 #5
    Can I use 100 N for the F in F = -kx?
     
  7. Feb 1, 2015 #6

    haruspex

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    Yes.
     
  8. Feb 2, 2015 #7
    If I put everything in meters, I get 40 cm. Here is my calculation:

    F = -kx
    -100 = -1000(x-0.3)
    x = .4 m or x = 40 cm.

    Did I set up the math correctly? I am not sure whether it should be x-0.3 or 0.3--x
     
  9. Feb 2, 2015 #8

    haruspex

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    In the equation F = -kx, what does x represent?
     
  10. Feb 3, 2015 #9
    Ok, my mistake. It should be -100 = -1000(.3x) = 33 cm. If this is correct then I understand.
     
  11. Feb 3, 2015 #10

    haruspex

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    No. Please answer my question regarding what x represents in that equation.
     
  12. Feb 3, 2015 #11
    X is the change (displacement) from its resting position.
     
  13. Feb 3, 2015 #12

    haruspex

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    Right. You are told F and k. Calculate x.
     
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