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Relationship between period and time in oscillators

  1. Apr 1, 2017 #1
    1. The problem statement, all variables and given/known data
    If the amplitude of a weakly damped oscillator decreased to 1/e of its initial value after n periods, show that the frequency of the oscillator must be approximately [1 − (8π2n2)-1] times the frequency of an undamped oscillator with the same natural frequency.

    2. Relevant equations
    The equation for a weakly damped harmonic oscillator is x(t) =Ae-βtcos(ω1t-δ) where ω1 = sqrt(ωo22)
    A = amplitude
    β = decay constant
    ω1 = period for damped oscillator
    ωo = natural frequency
    T = period
    t = time
    δ = phase angle
    3. The attempt at a solution
    Since the amplitude was initially Ae-βt and finally 1/e, I solved for t=1/β or β=1/t. I then took the equation ω1 = sqrt(ωo22) and did an expansion to the second term which gave me that ω1o(1-β2/2ωo2). Ridding of β2 for 1/t gives me ω1o(1-1/2ωo2t2). I can then replace ωo with 2πn/T and then I would almost have the correct answer, but for that t= T which doesn't make sense. Also as an aside do I not have to include the phase angle (so I can make it zero) since I'm not measuring the oscillator in reference to another one? Or is it there due to impedance or something?
     
    Last edited: Apr 1, 2017
  2. jcsd
  3. Apr 1, 2017 #2

    vela

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    You're introducing ##n## in the wrong place. If ##1/\beta## is the time for ##n## periods, you can say ##1/\beta = n(2\pi/\omega_1)##.
     
  4. Apr 1, 2017 #3
    Okay, can you explain why that is?
     
  5. Apr 1, 2017 #4

    vela

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    How are the angular frequency of the damped oscillator ##\omega_1## and the period ##T## related?
     
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