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Harmonic motion with damping

  1. Nov 12, 2015 #1
    1. The problem statement, all variables and given/known data
    An object of mass 0.2kg is hung from a spring whose spring constant is 80N/m. The object is subject to a resistive force given by -bv, where v is it's velocity in meters per second.

    If the damped frequency is √(3)/2 of the undamped frequency, what is the value of b?

    2. Relevant equations
    F=ma
    ω=√k/m

    3. The attempt at a solution
    I tried to write the sum of the forces of the system and got ∑F=-kx-bv=ma
    I rewrote it as -kx=b(dx/dt)+m(d^2x/dt^2)

    Now I don't have much experience with differential equations but I know the solution is x(t)=Ae^(γt)cos(ωt) where γ=(-b/2m). I also know that the damped frequency is (√(3)/2)√k/m given from the problem. I not sure where to go from here. I am supposed to use the solution and solve for b? Any help would be appreciated.
     
  2. jcsd
  3. Nov 13, 2015 #2

    rude man

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    What is the value of ω in terms of k, m and/or b? You know γ(b,k,m) but you don't know ω(b,k,m)?
     
  4. Nov 13, 2015 #3
    So ω in terms of k,m, and b would be √((ω^2)-(γ^2)) right?
     
  5. Nov 14, 2015 #4

    rude man

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    Going back to your 1st post, you are mixing up two ω's. One is the natural frequency ωn = √(k/m). the other is the damped frequency which is ωd. The ω in your cos argument should be the latter. The idea is that ωd < ωn as your problem statement gives. Don't use ω again, use the two above.
    Having said that, to answer your question
    is correct IF you use the right omegas. You can't say x = x + a, a ≠ 0, can you?
     
    Last edited: Nov 14, 2015
  6. Nov 15, 2015 #5
    Yeah I agree with that. I seem to be getting the 2 omega's confused with each other. So my solution should actually be expressed as x(t)=Ae^(γt)cos(ωdt), where ωd refers to the damped frequency. Sorry for making it look messy but I wasn't sure how to write the subscript d. Plugging in √((ω^2)-(γ^2)) for ω damping I can solve for my value of b. Is that the right idea?
     
  7. Nov 16, 2015 #6

    rude man

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    1. You're still using ω instead of wd or ωn. Don't.
    2 . Picking the correct omegas, rewrite your equation; this time make it a real equation with an = sign and everything.
    3. It's easy to make subscripts or superscripts. See the "x2" and the "x2" on the toolbar where you got your ω?
     
  8. Nov 19, 2015 #7
    Sorry for the late reply. Yes I found it, thanks for pointing that out.
     
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