NCERT Solutions for Class 9th Science Chapter 10 Let us explore — Activity 10.2

Book page 186 Updated on2026-09-08

Q1.

Strike one of the prongs of the tuning fork gently against the rubber pad (Fig. 10.4b) and bring it close to your ear. Do you hear a sound? (Take care not to strike the tuning fork against a hard surface).

Answer

Yes — a steady, pure humming note.

The blow sets the two prongs vibrating towards and away from each other. Each outward swing compresses the air near the prong and each inward swing rarefies it, so a train of compressions and rarefactions travels to your ear.

Why a tuning fork is used in sound experiments: unlike a voice or a bell, it gives almost a single frequency (one clean note). Its frequency is fixed by the length, thickness and material of the prongs, so it does not change however hard you strike it — striking harder only makes it louder, not higher.

Tip: strike it on a soft rubber pad, never on a hard desk or the edge of a table. A hard surface can bend or chip the prongs and permanently change the fork's frequency.

Q2.

Now, gently touch a water surface with one of the vibrating prongs of the tuning fork. Do you see waves forming on the surface of water?

Answer

Yes — ripples spread outward from the touching prong, and a fine spray may even be thrown up.

Why this matters: you cannot see the prong vibrating, but you can see what it does. Each time the prong moves down it pushes the water down; each time it moves up the water rushes back. That repeated push and release is what launches the ripples. So the ripples are visible evidence that the prongs really are vibrating — and therefore that the sound you heard was produced by vibrations.

Check it yourself: touch the water with a fork that has not been struck. There are no ripples. Strike it and touch again — ripples appear at once, and the humming note gets quieter because the water is draining energy from the prongs.

Q3.

Repeat step 3 a few times while bringing the prongs of the tuning fork near your ear in different orientations. Do you hear the sound?

Answer

Yes — the sound is heard in every orientation, though its loudness changes a little as you turn the fork.

Why it happens: the vibrating prongs disturb the air all around them, not in one line only. Each disturbed particle collides with neighbours in all directions, so compressions and rarefactions spread out from the source as roughly spherical waves (Fig. 10.10). Wherever you put your ear, some part of that expanding wave reaches it.

The change in loudness with orientation happens because the two prongs move in opposite directions, so in a few special directions their disturbances partly cancel. The general lesson stands: sound propagates in many directions from a source at once.

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