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Damping Constant

  1. Jan 13, 2009 #1

    I am working on a problem with a free hanging mass attached to a spring and damper in parallel. I believe my system equation is as follows:

    my'' = mg - by' - ky

    My questions:

    1) It has been a while since i've taken this course and just wanted to confirm I wrote my system equation correctly...for some reason I remember the spring and dampers in parallel add (opposite of those in circuits).

    2) Where can I find or calculate a value for damping coefficient, b, assuming damping is caused by seawater ?
  2. jcsd
  3. Jan 13, 2009 #2
    Your equation is correct :D
    It is basically a 1D motion, of a damped driven harmonic oscillator..

    But if you want to do it in sea water, then it wont be correct, since you have to take into account the flow around the hanging mass , which is not so easy. For a sphere it is easy to calculate the frequency shift due to the flow, because of the symmetry, but for another assymetric object its not so easy...
  4. Jan 14, 2009 #3
    You could easily determine the damping coefficient from a simple home-made experiment.
    - Research what the salinity of sea water is.
    - Mix table salt and water together to the same concentration.
    - Given that you know the mass of the object, m.
    - Take measurements of travel distance and time for the following tests:
    - First, suspend the weight onto the spring in open air (not sea water) to determine the spring constant, k.
    - Second, suspend the weight onto the spring in sea water to determine the damping coefficient, b.

    For comparison, you might find from Google the damping coefficient for an object in pure water. I would imagine the damping b will be similar (but less) as that for sea water.
    Last edited: Jan 14, 2009
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