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Consider a single degree-of-freedom system with viscous damping excited by a harmonic force. At resonance, the phase angle (in degrees) of the displacement with respect to the exciting force is

1. 0
2. 45
3. 90
4. 135

Correct answer: 90

Solution

For a single degree-of-freedom viscously damped system under harmonic forcing, the displacement lags the force by a phase angle. At resonance, this lag is $90^\circ$. The provided answer key is incorrect.

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