Resonance

A resonant frequency occurs whenever there is the regular interchange of forms of energy with little damping due to friction etc. A simple pendulum and water in a rocking fish tank exchange PE and KE. Air in a car with one slightly open window, electrons in a klystron tube, a ringing bell, and light in a laser cavity all resonate.

Note the four antinodes and nodal lines.

All resonant systems have a natural oscillation frequency, (and higher harmonics). All musical instruments resonate. Speech is formed in large part by altering the shape of the mouth which is a resonant cavity.

Cars are designed to avoid excitation at resonant frequencies by engine vibration and road noise. Bridges are designed to avoid resonance at large amplitudes due to waves, or the wind action that destroyed the Tacoma Narrows bridge in 1940.

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