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Homework Help: Damped Oscillation and amplitude

  1. Sep 11, 2007 #1
    1. The problem statement, all variables and given/known data

    Given: "In a science museum, a 110 kg brass pendulum bob swings at the end of a 15.0-m-long wire. The pendulum is started at exactly 8:00 a.m. every morning by pulling it 1.5 m to the side and releasing it. Because of its compact shape and smooth surface, the pendulum's damping constant is only 0.010 kg/s."


    (1) At exactly 12:00 noon, how many oscillations will the pendulum have completed?

    (2) And what is its amplitude?

    2. Relevant equations

    None Given

    3. The attempt at a solution

    I used the equation x=A[tex]_{}0[/tex] e [tex]^{}-(b/2m)t[/tex] cos( [tex]\varpi[/tex] [tex]\acute{}[/tex] t+[tex]\phi[/tex])

    I used the first bit of the equation to find the exact amplitude t(x) when x=14400 (x=A[tex]_{}0[/tex]e[tex]^{}-(b/2m)t[/tex] to find the amplitude)

    But the trouble I'm having is the number of oscillations in the 4 hour period.

    I took the angular frequency ([tex]\varpi\acute{}[/tex]) and multiplied that by the number of seconds (14400), but the resulting answer was incorrect. Since [tex]\phi[/tex]=0, taking the cosine of ([tex]\varpi\acute{}[/tex]) gives another answer, but I'm not confident that it is the correct answer, and I don't want to stab in the dark until I get it right.

    I'm a bit stuck.

    Since this is damped oscillation, and the initial period is greater than one second, the number HAS to be less than 14400.

    Any help? Am I on the right track? Is there something I'm missing?

    Note: It doesn't seem that the latex is putting superscripts in the correct locations, so please bear with me.
    Last edited: Sep 11, 2007
  2. jcsd
  3. Sep 11, 2007 #2


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    Homework Helper

    have a look at the thread in intro physics.
  4. Sep 11, 2007 #3
    Thank you! All solved.
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