Resonance

The effects are all due to a phenomenon called 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 swings 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. >


The opera singer can shatter the wine glass because the frequency if the note sung by the singer is equal to the natural frequency of the wine glass when it is hit – the vibrations build up and eventually break the glass.
In earthquakes some of the building swill vibrate a lot because their natural frequency if equal to that of the movement of the ground beneath them. In earthquake zones It is important to try and design buildings where this is not the case. (Find out about the resonance effects in the bowl of mountains during the Mexico City earthquake.)


A simple demonstration of mechanical resonance is possible using the apparatus shown in the diagram.

Resonance occurs when the strip vibrates violently.

You can investigate the resonant frequency of the apparatus varies by changing the mass of the plasticene ball and the length of the metal strip.