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The amplitude of free vibration of a damped structure decays

  • A.linearly with time
  • B.in proportion to the square of the elapsed time
  • C.not at all once motion has begun
  • D.exponentially with time

Answer: D

Correct answer: exponentially with time

The free vibration solution contains the factor \( e^{-\xi\omega_n t} \), so the envelope decays exponentially.

The consequence is that the ratio of successive peaks is constant, which is exactly what makes the logarithmic decrement a well-defined quantity: taking the logarithm of a constant ratio gives the same value for every pair of peaks, so several cycles can be averaged. Linear decay would give a constant difference instead, and no such measurement would be possible.

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