Resonance

Resonance occurs when a driven oscillator is made to vibrate at the same frequency as a driver oscillator. Resonance effects are characterised by vibrations with large amplitude.

Here is a link to  Barton’s pendulum. You will see this in class

Think about a child being pushed on a swing.

The pusher (driver) pushes at the same frequency as the natural frequency of oscillation of the swing (driven). The result is that the swing oscillates with large amplitude, since the energy it needs to gain amplitude is being supplied at exactly the right time in the cycle.

These resonance effects are common. The bus side vibrates as it pulls away, since the metal skin of the bus has the same natural frequency as the engine at low revs. Imperfectly balanced washing machines ‘walk’across the floor during the spin cycle since the frequency of the vertical component is the same as the natural frequency of vibration of the retaining springs, causing vibrations with large amplitude Sometimes , these can be disatrous.  Check out the video of the famous collapsing bridge over Tacoma Narrows in Washington State, called “Galloping Gertie”. Amazing! The bridge collapsed in November 1940 because of a resonant effect between the material in its construction and the wind whistling down the gorge, rather like blowing a reed wind instrument to make a loud sound.

Chladni figures are complex resonant vibrations made on a flat sheet. Originally, Robert Hooke (yes, him again!) experimented with drawing a violin bow across flour-covered glass plates. Ernst Chladni  (photo) used the same trick with metal sheets, subsequently with a magnetic oscillator in order to fine-tune the frequencies more accurately. They’re quite fun to watch , so check out the videos if you want to know more. The rest of the animations here are worth a look, also.

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About John Vagabond

I have taught physics and math all over the world
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