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  Physical scientists describe many varied examples of resonant phenomena. One example often quoted is the playground swing, which must be pushed at the correct intervals to attain a good amplitude of swing. Another is the behaviour of water in a vessel—the water in a bath slops out if it is swished back and forth at the right frequency. A diver bounces on the board at the correct frequency to build up a good ‘spring’. All such events depend upon resonance.

All objects have what is known as a natural frequency. This is the frequency at which they will happily oscillate, and which, if given a sudden impulse, they will settle down into. An excellent example of this is a pendulum: given a push, it will begin to swing at its natural frequency.

Resonance occurs when we push or strike an object at or close to its resonant frequency. Pushing a swing at random intervals will not achieve very much, but as soon as we find the resonant frequency, the swing responds and goes much higher. The closer the applied or driving frequency approaches to the resonant frequency, the more the object responds and oscillates.

Resonance is vital to musical instruments. Sound boxes are designed to resonate at the frequencies produced by the strings within them, thus producing a better sound. Strings will vibrate if a note at their natural frequency is played close to them.

The resonant frequency of buildings and similar structures must be investigated carefully to avoid damage, as a building may shake violently if a force is applied to it at the right frequency. The best-known example of this was the Tacoma Narrows bridge, in the state of Washington. The wind, gusting at a steady rate, caused the bridge to twist like a piece of ribbon until it was totally destroyed. Bridges nowadays have shorter spans to alter their natural frequency and prevent similar disasters. JJ



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