What is resonant frequency, and how do things like bridges collapse because of it?
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Someone’s already mentioned a swing, so I’ll give another basic example.
Find a bottle, the bigger the better. Now half fill it with water, turn it on its side, and try to rock the bottle back and forth to create the biggest wave you can. You should notice some things:
* The size of the waves doesn’t affect the frequency of the waves. In other words, the system (the half-filled bottle) has a natural frequency determined that is independent of the amplitude in each wave.
* When the frequency with which you rock the bottle *matches* the natural frequency of the waves, your wave grows bigger and bigger each period.
Boom, that’s resonant frequency. If you rock the bottle at some other frequency, then that dissonance in the frequencies means your motion is taking away energy from the system just as often as it is adding energy, and so your wave doesn’t reliably grow. It’s only when you time your actions in a way that each action is adding energy that your wave grows.
The exact same phenomenon is apparent, say, when singers break glasses by singing at a high pitch. A wine glass has a *natural frequency* that is determined only by its shape and the speed of sound in glass. When singers match that frequency, each pressure wave (of which there are thousands per second) only adds to the magnitude of the pressure waves within the glass. Eventually, the glass cannot accommodate the stress, and so it breaks.
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