NCERT Solutions for Class 9th Science Chapter 10 Let us experiment — Activity 10.6

Book page 191 – 192 Updated on2026-09-08

Q1.

Produce a loud sound near the bowl without touching it. Observe the grains on the sheet. Does the sound have any effect on the grains?

Answer

Yes — the grains jump, dance and shuffle across the stretched sheet, even though nothing has touched the container.

Why it happens, step by step:
1. The struck metal plate vibrates and sends compressions and rarefactions out through the air.
2. These reach the tightly stretched sheet. A compression presses on it, a rarefaction releases it — so the sheet is pushed and released many times a second and starts to vibrate.
3. The vibrating sheet throws the grains resting on it up into the air.

Since the grains have been lifted against gravity, work has been done on them. That work could only have come from the sound, so the activity proves that sound carries energy. When the source vibrates, it transfers energy to the medium; the vibrating particles of the medium collide with others, and the energy is relayed onward.

Tip: spread the grains evenly and do not let them clump — a clump is heavier and needs more energy to lift, so the effect becomes hard to see.

Q2.

Repeat step 4 with different sources of sounds and observe the effect on the grains. You can try increasing or reducing the volume of sound. Try with different grains.

Answer

The louder the sound, the higher and more violently the grains jump. With a soft sound they barely quiver; with a very loud one they leap right off the sheet.

What you change	What you observe	Reason
Strike the plate harder (louder sound)	Grains jump much higher	Larger amplitude → more energy carried by the wave → the sheet is displaced further
Move the source further away	Grains move less	The same energy is spread over a larger area, so the intensity at the sheet falls
Use light grains (semolina, chalk powder) instead of heavy ones (salt, rice)	Lighter grains jump higher and more easily	Same energy delivered to a smaller mass gives a larger displacement
Loosen the stretched sheet	Response becomes weaker and duller	A slack sheet does not follow the rapid pushes of the sound wave faithfully

Why loudness and amplitude go together: striking the plate harder does not change how many times per second it vibrates (the frequency is the same, so the note sounds the same). It changes how far the plate swings, which makes the density change in each compression bigger — a larger amplitude. A larger-amplitude wave carries more energy, so more energy reaches the sheet each second and the grains fly higher.

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