What is a torsional wave?

If the energy of a resonant standing wave is large enough, a crystal wine glass can shatter quite easily, but when the amplitude is smaller, the wine glass can produce a sound instead. Take, for example, a person rubbing his finger around the moist lip of a crystal wine glass. The glass seems to sing or hum. The humming is caused by the rubbing of the wet finger on the glass that causes it to have a standing wave pattern. The resonating glass generates enough energy to vibrate the surrounding air and create a steady humming sound. In 1761 Benjamin Franklin (1706-1790) invented the glass harmonica (that he called the armonica) in which wine glasses of various sizes were fastened to a rotating shaft. The musician rubbed his fingers on the appropriate glass to play music. Wolfgang Amadeus Mozart and almost 100 other composers wrote music for the glass harmonica.

A torsional wave, such as the wave that the Tacoma Narrows Bridge created, is a wave that is not only displaced vertically, but twists in a wave-like fashion as well. The torsional wave on the Tacoma Narrows Bridge achieved resonance in two orientations. The first resonance was seen as the undulating movement that took place over the length of the bridge, while the second resonance, seen as the twisting motion, occurred from side to side on the bridge.


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