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Periodic Motion question

  1. Apr 8, 2005 #1
    Here is my problem:
    A hard-boiled egg of mass 45.0 g moves on the end of a spring with force constant k = 24.7 N/m. Its initial displacement is 0.290 m. A damping force F = - bv acts on the egg, and the amplitude of the motion decreases to 0.120 m in a time of 5.10 s.

    I need to find the magnitude of the dampening constant b.

    I am completely clueless on how to approach this question.

    edit: corrected some stuff.
    Last edited: Apr 8, 2005
  2. jcsd
  3. Apr 8, 2005 #2
    Me too, since I do not know what you mean by 'the force of B'.
  4. Apr 8, 2005 #3
    sorry i guess i typed it wrong. the dampening force:

    F = -bv

    I need to find the magnitude of the dampening constant b.
  5. Apr 8, 2005 #4
    [tex] x(t) = x_{m} e^{\frac{-bt}{2m}} cos( \omega t) [/tex]

    where Xm is the initial displacement
    b is hte damping force
    m is the mass
    omega is the angular frequency [tex] \omega = \frac{2 \pi}{T} [/tex] where [tex] T = 2 \pi \sqrt{\frac{m}{k}} [/tex]
  6. Apr 8, 2005 #5
    for some reason that is not working for me.
  7. Apr 8, 2005 #6
    perhaps you are not using your numbers correctly

    initla displacement Xm = 0.290 m
    k = 24.7 N/m
    X(5.10) = 0.120 m
    t = 5.10s
    m = 45g = 0.045 kg
    and omega = [itex] \sqrt{\frac{m}{k}} [/itex]
    it's blind substitution, really
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