Natural frequency - Wikipedia
https://en.wikipedia.org/wiki/Natural_frequency
Free vibrations of an elastic body, also called natural vibrations, occur at the natural frequency. Natural vibrations are different from forced vibrations which happen at the frequency of an applied force (forced frequency). If the forced frequency is equal to the natural frequency, the vibrations' amplitude increases manyfold. This phenomenon is known as resonance. A system's normal mode is defined by the oscillation of a natural frequency in a sine waveform.
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