The response of a linear oscillator to uniform forcing as a function of frequency. At frequencies below the resonance frequency (long periods) the response is direct (the phase is close to 0°) , ie the oscillator moves in the direction of the forcing. At frequencies above the resonance frequency (short periods) the response is inverse, ie the oscillator moves against the direction of the forcing (the phase approaches 180°). At resonance the response is 90° out of phase, ie the oscillator is at rest when the forcing is maximum and goes through maximum movement when the forcing goes through zero.

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