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SPRING CONSTANT- simple harmonic motion

  1. Aug 20, 2008 #1
    I'm investigating damped oscillations using a mass held between two springs.
    In the experiment, I measured the spring constant using a method which now doesn't make sense.

    I measured it by hanging masses from ONE vertical spring and graphing the force exerted by spring (mass times 9.8) against the elongation of spring

    I think that's wrong. I think I should have used a pulley or something like this:
    the glider is held between two springs. Record its equilibrium
    2. Attach a piece of audio tape to the glider and lay it across the “air
    pulley” with a small mass suspended on the end of the tape.
    3. Measure the displacement of the glider from equilibrium for 4
    different hanging masses.
    4. Graph the weight of the hanging mass (y axis) vs. the measured

    But I don't even understand how this method works. What does it mean by attaching a piece of tape to the glider and laying it across the 'air pulley'?

    My second and main concern is that because I cannot redo the experiment, I have to somehow use the k values that I've measured using the masses hanging on the single spring. How can I fix the k values to get one that will equal the k value resulting from two springs, one on each side of the mass?

    any input appreciated!
  2. jcsd
  3. Aug 20, 2008 #2

    Doc Al

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    Staff: Mentor

    That's a perfectly fine way to measure the spring constant. I assume you measured the spring constant of each spring?
    How do you find the effective spring constant of the two springs together? Hint: With a spring attached to each end, is the effective spring constant greater or less than each individual spring constant?

    To figure that out, imagine the mass between both springs in equilibrium. Pull it a distance X to one side. What net force (due to both springs) acts on the mass? Set that net force equal to KX, where K is the effective spring constant.
  4. Aug 20, 2008 #3
    I think I know what you're saying Doc. If you put two springs in seires then the effective spring constant is k times two

    But my situation is where the mass is held BETWEEN the two springs.
  5. Aug 20, 2008 #4
    So I think one way to get the effective spring constant is to use a pulley somehow.
  6. Aug 20, 2008 #5
    unless there's an equation that relates the individual spring constants to the effective spring constant when the mass is held between two identical springs
  7. Aug 21, 2008 #6

    Doc Al

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    Staff: Mentor

    You have it backwards.
    I know. :wink:
    Huh? No need to complicate things by adding a pulley to the mix.

    The way to get the effective spring constant is to do what I said in my last post. (You'll have to apply Hooke's law.)
    Of course there is. But rather than look for such an equation in a book (you might get lucky!), just figure it out for yourself by doing what I suggested.
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