NCERT Solutions for Class 9th Science Chapter 10 Let us explore — Activity 10.1
Book page 185 Updated on2026-09-08
Q1.
Holding the box steady with one hand, pluck the rubber band with a finger. Do you hear any sound?
Answer
Yes, a clear twang is heard.
Plucking pulls the stretched band away from its mean position. When you let go, the tension pulls it back, it overshoots, and it keeps moving to and fro — it vibrates. The vibrating band pushes the air next to it forwards and backwards, sending compressions and rarefactions towards your ear.
Why the box matters: a bare band moves very little air. Stretched across the open box, it also sets the air inside the box and the cardboard itself vibrating, so far more air is disturbed and the sound is much louder. This is exactly why a sitar, veena or guitar has a hollow body.
Q2.
Pluck the rubber band again and watch it carefully. Is it vibrating?
Answer
Yes. Just after plucking, the band looks blurred — you see a widened, fuzzy band instead of a sharp line, because it is moving to and fro faster than your eye can follow.
vibration = periodic to-and-fro motion (oscillation) about a mean position
the blur width across the band ≈ twice the amplitude of the vibration
Check it yourself: touch the plucked band very lightly with a fingertip. You feel the buzz — and the sound dies at once, because your finger absorbs the vibrational energy.
Q3.
Wait till the rubber band stops vibrating. Do you still hear the sound?
Answer
No. The moment the band comes to rest, the sound stops.
Why it happens: the sound was being fed by the band's own vibrational energy. Each swing hands a little energy to the air (and a little is lost as heat inside the stretched rubber). Once that store is used up the band stops, no new compressions are made, and there is nothing left to reach your ear.
This one observation is the whole conclusion of the activity: sound is produced by vibrations; no vibration, no sound.
Q4.
Change the tension in the rubber band by stretching it more or loosening it slightly and plucking it each time. Does the sound change? What changes do you notice?
Answer
Yes, the sound changes. Stretch the band tighter and the note becomes shriller (higher pitch); loosen it and the note becomes deeper (lower pitch).
What you do
How the band vibrates
Frequency ν
Time period T
What you hear
Stretch it more (higher tension)
Snaps back faster
higher
shorter
shriller, high pitch
Loosen it (lower tension)
Returns sluggishly
lower
longer
deeper, low pitch
Pluck harder
Larger swing about the mean position
unchanged
unchanged
louder (larger amplitude)
Why it happens: a tighter band is pulled back to its mean position by a bigger restoring force, so it completes each to-and-fro trip in less time. Since ν = 1/T, a shorter time period means a higher frequency, and higher frequency is heard as higher pitch.
Tip: notice that tension changes the pitch while how hard you pluck changes the loudness. That is the difference between frequency and amplitude, in your own hands. It is also how a sitar or tanpura is tuned — by turning the pegs to change the tension.
Q5.
Remove the rubber band from the box. Stretch it between two fingers and pluck it near your ear. Is the sound still produced? Is it as loud as before?
Answer
Sound is still produced, but it is much fainter than before.
Why it happens: the source of the sound is the vibrating band, and it is still vibrating — so sound must be produced. But a thin band by itself sweeps only a small volume of air, so it transfers very little energy per second to the surrounding air. On the box, the band's vibrations were passed to the box and to the air column inside it, and that much larger vibrating surface set far more air moving. Larger amplitude of the air's density oscillation means more energy carried, hence greater loudness.
Try This: hold the plucked band against a wooden desk or a steel tumbler. The sound jumps in loudness again — the desk or tumbler is acting as a sounding board, exactly like the body of a violin.