Science 8th grade
UNIT 11: Sound 11.2 Looking at vibrations
Science 8th grade
UNIT 11: Sound 11.2 Looking at vibrations
Amplitude and frequency of vibrations
When you pluck a guitar string, it vibrates very rapidly. It may vibrate hundreds or thousands of times cach second. This is too fast to see clearly.
The picture shows one way to observe slow vibrations. A metre rule is clamped to the bench. A weight is taped to the free end.
When you pull the end downwards and let it go, the weight vibrates up and down. If you do this with a short ruler, you can make a 'twanging sound.
The picture shows the amplitude of the vibration. It tells you the maximum distance the vibrating object moves from its rest position before it started vibrating.
The number of vibrations per second is called the frequency of vibration. If an object vibrates 20 times each second, we say that its frequency is $20Hz$. The symbol Hz stands for hertz, the unit of frequency.
1 hertz = $1Hz$ = l vibration per second
To measure the frequency of a vibration, you may have to time a large number of complete vibrations - say, 20 or 50 - and then calculate how many vibrations there are in ls. This is like taking your pulse; you can't time a single heartbeat accurately.
Questions
1) a: If a guitar string vibrates 250 times each second, what is its frequency?
b: If a drum skin vibrates with a frequency of $100Hz$, how many times docs it move up and down cach second?
2) If a bird flaps its wings up and down 50 times in $20s$, what is the frequency of its flapping?
Activity 11.2 (Studying vibrations)
SE: You are going to investigate the vibrations of a ruler which has been clamped at one end. Firstly, you will need to decide how to measure the frequency of its vibrations. Discuss your ideas with your partners and then share your best idea with the rest of the class.
Secondly, choose a question to investigate. Write a plan and check it with your teacher before you start.
• How will the frequency of the vibrations change if you make the ruler longer or shorter?
• How will the frequency of the vibrations change if you make the ruler vibrate up and down with a greater amplitude?
• How will the frequency of the vibrations change if you change the weight attached to its end?
You may be able to think of a question of your own to investigate. Before you carry out your investigation, write a prediction: what do you think you will find? Give a reason to support your prediction.