Vibrations and To-and-fro Motion
Your textbook's first big idea is that sound is produced by vibration (കമ്പനം). Pluck a rubber band, strike a tuning fork or speak – something is moving to and fro very quickly. This page shows what “to-and-fro motion” means, how it differs from straight-line motion (നേർരേഖാചലനം), what one complete vibration is, and how vibrations can be faster or slower. You will use swings, a pendulum, a ruler on the edge of a desk and a rubber band.
- Nothing from earlier classes – just a ruler, a rubber band and a little string if you want to try things at home.
- Dividing one number by another (for the “go further” part).
1. Two kinds of motion
A coconut falling from a tree moves in one direction along a straight line – from the tree to the ground. A bus on a straight road does the same. This is straight-line (linear) motion (നേർരേഖാചലനം): the object keeps going and does not come back.
A swing is different. It moves forward, comes back, moves forward again – to and fro about a middle position where it would hang at rest. A clock pendulum, a plucked string and the prongs of a tuning fork move in the same way. Such to-and-fro motion is called oscillation; when it is quick and small, as in a plucked string, we call it vibration (കമ്പനം).
Look at the trail of the red dot. In straight-line motion the trail keeps getting longer in one direction. In to-and-fro motion the dot goes over the same short path again and again, on both sides of a middle position.
2. One complete vibration
Every to-and-fro motion has three special places:
- the middle (rest) position, O – where the object stays when it is not moving;
- two extreme positions, A and B – the farthest points on each side, where the object stops for an instant and turns back.
One complete vibration (or one oscillation) is a full round trip: A → O → B → O → A. The object passes the middle position twice during one vibration. The time taken for one complete vibration is called the time period.
The pendulum swings in real time. Try (1) a longer string – the swings become slower; (2) pulling it farther – the swings become wider, but the time for one vibration hardly changes. (The numbers come from the pendulum formula you will meet in higher classes, with g = 9.8 m/s².)
During Onam, children tie an oonjal (swing) to a strong branch of a mango tree. Give it one push and it swings to and fro by itself for a long time, each swing taking about the same time. A swing with long ropes swings slowly; a short one swings quickly. The pendulum of an old wall clock in a tharavad house does exactly the same, and its steady to-and-fro motion keeps the time.
3. Vibrations you can feel and hear
A swing oscillates slowly – about once every few seconds. Many objects vibrate much faster, so fast that the eye sees only a blur. Your textbook's activity with a rubber band (Fig 8.2) shows this: stretch a band over a pencil box resting on two pencils and pluck it. You hear a sound, and the band moves to and fro rapidly – it vibrates. Touch it and the vibration stops – and so does the sound.
The animation is shown in slow motion. Pluck a tighter band: it vibrates faster and the sound is shriller (higher). Hold it: no vibration, no sound. This is the heart of the chapter: no vibration → no sound.
Press one end of a steel ruler firmly on the desk and flick the free end. A long free part flaps slowly; a short one vibrates fast and gives a buzzing note. The numbers are approximate (for a thin steel ruler about 1 mm thick), and the animation is slowed down 20 times so that you can see the motion.
- Motion in one direction along a line is straight-line motion (നേർരേഖാചലനം).
- Repeated motion to and fro about a middle position is oscillation; quick, small oscillation is vibration (കമ്പനം).
- One complete vibration = A → O → B → O → A.
- Every object that makes sound – a source of sound (ശബ്ദസ്രോതസ്സ്) – is vibrating. Stop the vibration and the sound stops.
4. Go further: how fast is a vibration? (frequency)
To compare fast and slow vibrations, count how many complete vibrations happen in one second. This number is called the frequency, and its unit is the hertz (Hz): 1 Hz = 1 vibration per second. A swing has a frequency of less than 1 Hz. A school tuning fork (ട്യൂണിങ് ഫോർക്ക്) is often marked “256 Hz” or “512 Hz”: its prongs vibrate 256 or 512 times every second. Your ears can hear vibrations from about 20 Hz to 20 000 Hz (approx.; the upper limit falls with age).
Frequency is not in your Class 9 textbook chapter – it is here only to help you understand “faster” and “slower” vibrations. You will meet it properly in higher classes.
Worked example 1 · The swing
Q. A girl on an oonjal completes 12 vibrations in 30 seconds. Find (a) the time for one vibration, and (b) the number of vibrations in one second (the frequency).
For (a), share the 30 seconds equally among the 12 vibrations. For (b), share the 12 vibrations among the 30 seconds.
Worked example 2 · The tuning fork
Q. The prongs of a tuning fork marked 256 Hz make 256 vibrations every second. (a) How many vibrations do they make in 5 seconds? (b) How long does one vibration take?
(a) 256 vibrations every second, for 5 seconds – multiply. (b) 256 vibrations share one second – divide 1 by 256.
Tie a small stone to a 1 m string and let it swing. Count 10 complete vibrations while someone times them with a phone stopwatch. Divide the time by 10. You should get close to 2 s. Now use a string 25 cm long – you should get close to 1 s. Write your readings in your science diary (സയൻസ് ഡയറി).