Spark image

Resonance

A radio playing loud music on a shelf may make a glass vibrate that is also on the shelf
If you push someone on a swing it is possible to make them swing higher and higher
If the load in a washing machine is uneven it is possible to get large vibrations when it is spinning
A singer can break a wine glass if they sing the right note very loudly close to it
The body panels or the bonnet of a car may vibrate strongly at certain speeds on a bumpy road
A baby in a baby bouncer can build up a really big bounce!
If you sing or play near a piano you can make some of the piano strings vibrate
In an earthquake some buildings shake more violently than others
The Tacoma Narrows suspension bridge fell to pieces after violent vibrations in a high wind
Food containing water cooks in a microwave oven
A skipping rope can be made to vibrate strongly at only certain frequencies

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.

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

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.



 
 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD
 
 
 
© Keith Gibbs 2020