# Vibrations and Waves

Music from a disco speaker travels to our ears as sound waves
A double bass can play lower notes than a violin
An earthquake or explosion sends shock waves through the ground
Music rooms in schools are often sound proofed
My son has to play his drum kit in an isolated hut
Ultrasound is often used to scan pregnant mothers
The design of concert halls is very important
The same pitch of note played on a guitar and a flute will sound quite different
A washing machine can vibrate badly if the clothes get out of balance

All these facts are to do with VIBRATIONS of one type or another. A vibration is a wobble, either from side to side or up and down.
In Physics we also call it an OSCILLATION.

## MECHANICAL VIBRATIONS

The following is a list of mechanical vibrations that you can easily try in the lab. For each experiment try to find out what affects the RATE of vibration.
1. Put a ruler over the edge of the bench and twang it.
2. Blow into a wooden organ pipe
3. Run your fingers round the top of a wine glass (clean them with meths first)
4. Fix a weight to a spring and then let it oscillate up and down
5. Play a saw with a bow (the side without the teeth!)
6. Make a simple pendulum with a mass on the end of a string
7. Play any stringed instrument
8. Fix a trolley on the bench between two supports using springs. Make the trolley oscillate.
9. Lay a slinky spring on the bench and vibrate one end
10. Half fill a plastic tank with water and move a wooden plunger up and down in it.

The RATE of vibration is called the FREQUENCY and is measured in HERTZ.

A frequency of 1 Hz is a rate of vibration of ONE oscillation per second.
High frequencies are measured in kilohertz (kHz) [1 kHz = 1000 Hz] or megahertz (MHz).[1 MHz = 1 000 000 Hz].
You may have met frequency scales before - on a piano or on a radio. The frequency of middle C is 256 Hz and that of FM radio about 100 MHz.

Large, heavy or slack objects vibrate slowly and have a low frequency
Small, light or tight objects vibrate quickly and have a high frequency.

PROBLEMS
1. A rule sticking out over the edge of a bench vibrates with a frequency of 10 Hz.
(a) How many vibrations per second does it make?
(b) How long does each vibration take?
(c) What would happen to the frequency if a little piece was cut off the end of the ruler?

2. Some car bonnets vibrate when driving over a rough road. How would you reduce the amount of vibration?

3. The Tacoma Narrows bridge collapsed after large vibrations in a high wind. How could the designers have prevented this? (Vibrations on a much less dramatic scale closed the Millenium Bridge across the River Thames in London until the bridge was modified).

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