**4.09 natural frequency****; undamped natural frequency****.** Frequency of free, undamped oscillation for a system. For a multiple-degree-of-freedom system, the natural frequencies are the frequencies of the normal modes of vibration. Unit, hertz (Hz).

Annotation For the equation of motion in Table 1, the undamped natural frequency is (1/2π)(*S*/*M*)^{1/2}. At this frequency the motion of the mass *M* lags the disturbing force by a phase angle of 90 degrees. For a more complicated system having several degrees of freedom, there will be as many frequencies at which the motion of the mass lags the disturbing force by a phase angle of 90 degrees as there are degrees of freedom; these are all undamped natural frequencies.

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