# Resonance

Resonance means the build up of vibrations of an object when it is vibrated at a certain frequency.

Objects can be made to vibrate when you push them in some way. Think about pushing a child sitting on a swing. You will find that as you push the swing will only swing further and further if you push at one certain rate. At any other rate and the swing's motion is irregular, it comes back and hits you as you are about to push again.

The only way to get the vibrations to build up is to push IN TIME with the swing's own rate of swing. To put it another way the frequency of your pushes must be the same as the frequency of the swing's movement.

The swing's own vibration frequency is called the natural frequency and the frequency of your pushes is called the driver frequency.

Resonance occurs when the driver frequency is equal to the natural frequency of the object

Famous examples of resonance in bridges were the collapse of the Tacoma Narrows bridge in the USA and the problems withe Millenium Bridge across the Thames in London (now solved!).

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