Resonance is a common phenomenon in nature. It is a violin string vibrating to produce a note, a leaf shaking unnaturally in the wind, or a wine glass singing as you rub its rim. Resonant frequency is the natural frequency at which an object wants to vibrate. When you match that frequency you will cause the object to resonate. If you sing hard enough at the correct note of a guitar string, you can get the string to continue vibrating, even after you stop. In certain objects, you can produce standing waves when you match their resonant frequency. Such is the case of a vibrating square plate. Below are the "modes" or resonant frequencies of vibration shown by sand sprinkled on a square plate. The lines in sand indicate where the plate is not moving.
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