Question

Consider a forced single degree-of-freedom system governed by x ¨ ( t ) + 2 ζ ω n x ˙ ( t ) + ω n 2 x ( t ) = ω n 2 cos ( ω t ) , 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 :

  1. 𝜔p < 𝜔d < 𝜔n

  2. 𝜔p = 𝜔d < 𝜔n

  3. 𝜔d < 𝜔n = 𝜔p

  4. 𝜔d < 𝜔n < 𝜔p

Show Answer

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 :  d x d r = 0

dx/dr = 2 (1 − r2). (-2r) + (2ζ)2.(2r) = 0

2 (1 − r2) = (2ζ)2

r2 = 1 - 2ζ2

here r = ωpn

r = ω p ω n = 1     2 ζ 2

ωp = 1     2 ζ 2

Now comparing wit with damped frequency:

ωd = 1     ζ 2

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 > ω> ωp

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