Question
Consider a forced single degree-of-freedom system governed by , where ζ and ωn are the damping ratio and undamped natural frequency of the system, respectively, while ω is the forcing frequency. The amplitude of the forced steady state response of this system is given by [(1 − r2)2 + (2ζr)2]-1/2, where 𝑟 = ω/ωn. The peak amplitude of this response occurs at a frequency ω = ωp. If ωd denotes the damped natural frequency of this system, which one of the following options is true?
Options :
𝜔p < 𝜔d < 𝜔n
𝜔p = 𝜔d < 𝜔n
𝜔d < 𝜔n = 𝜔p
𝜔d < 𝜔n < 𝜔p
Answer :
𝜔p < 𝜔d < 𝜔n
Solution :
𝑟 = ω/ωn
x = (1 − r2)2 + (2ζr)2
It is given that peak amplitude occurs at a frequency ω = ωp
it means, A → maximum, when the denominator (x) is minimum
Minimize x :
dx/dr = 2 (1 − r2). (-2r) + (2ζ)2.(2r) = 0
2 (1 − r2) = (2ζ)2
r2 = 1 - 2ζ2
here r = ωp/ωn
ωp =
Now comparing wit with damped frequency:
ωd =
We can clearly see that ωd > ωp
We already know that the natural frequency is greater than the damped frequency
ωn > ωd
Hence, we can say that:
ωn > ωd > ωp
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