Resonance
When small pushes arrive in step with an object's natural rhythm, its vibrations build up into a large swing.
Everything that can vibrate, from a guitar string to a bridge, has a natural frequency: the rhythm at which it swings if you disturb it and let go. Resonance is what happens when something keeps pushing it at that same rhythm. Each push arrives at just the right moment to add to the motion, so energy builds up and the vibration grows far larger than any single push could cause.
Pushing a child on a swing is the everyday example. Small shoves timed to the swing's own interval send it higher and higher; the same shoves at random moments achieve little. A singer holding exactly the right note can make a crystal wine glass vibrate so strongly that it shatters.
Resonance is also how most musical instruments and voices work. The strings and body of a violin, the air in a flute and the spaces of the human throat and mouth all resonate, picking out and strengthening particular frequencies.
The collapse of the Tacoma Narrows Bridge in 1940 is often presented as a classic case of resonance, but many engineers argue it was really aeroelastic flutter, in which the wind and the twisting deck fed energy into each other. Big vibrations are not always simple resonance, and in real objects damping, the steady loss of energy to friction, limits how large resonant swings can grow.
See it in action
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