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.