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Homework Help: Finding the spring constant of a mass in SHM

  1. Apr 23, 2014 #1

    This is question from A.P French vibrations and waves, problem 3-1 for anyone whom may have it

    An object of mass 1 g is hung from a spring and set in oscillatory motion. At t=0 the displacement is 43.785 cm and the acceleration is -1.7514 cm/sec^2. What is the spring constant?

    My attempt:

    I thought this problem straight forward and used

    ma= -kx

    using the numbers above but it gave the wrong answer? And I changed everything into S.I units I should also say

    The answer is 0.025 N/m or 25 dyn/cm

    I have been messing around with this for a while and gotten no where close to the correct answer!! Rather frustrating considering it is such a straight forward problem

    Any help would be great thanks in advanced
  2. jcsd
  3. Apr 23, 2014 #2
    Have you considered ALL the forces working on the object? Make sure For something to hang from a spring, what force is required?
  4. Apr 23, 2014 #3
    Gravity would be required!! But is it not true that simple harmonic motion obeys the same laws in the vertical direction as it does in the horizontal?
  5. Apr 23, 2014 #4
    The laws of motion (simple harmonic and otherwise) must consider ALL the forces.
  6. Apr 24, 2014 #5
    Would I just then have

    ma- mg= -kx
  7. Apr 24, 2014 #6
    I think you have included all the relevent forces. What answer do you get? Does it agree with the given answer? If not, is the given answer rounded? Are the initial conditions you posted the actual ones given in the problem (transcription errors are common)?

    Finally, it never hurts to check that all your displacements, accelerations and forces are consistent with your choice of coordinate system.

    Sorry I can't be more explicit. I have been admonished in the past for being too helpful.
  8. Apr 24, 2014 #7
    My answer came out to be

    26 dyn/cm
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