Resonance

Objects which oscillate freely (e.g. a mass on a spring or a pendulum) like to do so at a certain frequency. We call this its natural frequency. We have seen how the natural frequency of a mass on a spring depends on its mass and the stiffness of the spring.

There is another type of oscillation called a forced oscillation. This is when something is made to oscillate, e.g. when you shake something. The frequency making it oscillate is called the driving frequency.

I hope this guy doesn't wear false teeth!	Put a cup of coffee on a washing machine and watch what happens.	A motor makes the brush oscillate at a very high frequency.	During an earthquake everything is forced to oscillate.

Have you ever seen an opera singer break a glass just using their voice? (there are plenty of videos on YouTube) How is this possible?

 Something interesting happens when an object is forced to oscillate at a frequency close to or at its natural frequency. Large amplitude oscillations occur. This is called resonance and there are lots of examples where it can be useful or a nuisance.

If you flick or rub the rim of a wine glass it makes a note. Sing exactly the same note very loud near the glass and .....	The Tacoma Narrows bridge collapsed because the engineers who designed it did not realise that the wind blowing down the valley may cause it to resonate. There was a similar problem with the Millennium bridge in London.	Most musical instruments use resonance. In a clarinet the reed causes the air to vibrate and its natural frequency is changed by putting your fingers on different stops.

As the frequency approaches the natural frequency the amplitude increases. The grey line shows the effect of increasing the damping on the system.