Chapter – Sound (Class 9)
(B) VERY SHORT ANSWER TYPE
1. What causes sound?
Sound is caused due to vibrations of objects.
2. Complete the following sentence: Sound is produced by a body ________.
Sound is produced by a body in vibration.
3. There is no atmosphere on the Moon. Can you hear other people on the Moon's surface?
No, sound cannot be heard because there is no material medium.
4. Choose the correct word to complete the sentence: When sound travels in a medium, the ______ travels in the form of a wave.
The disturbance travels in the form of a wave.
5. Name the two kinds of waves in the form of which sound travels in a medium.
Longitudinal waves and transverse waves.
6. What is a longitudinal wave? In which medium—solid, liquid or gas—can it be produced?
A longitudinal wave is one in which particles vibrate parallel to the direction of propagation. It can be produced in solids, liquids and gases.
7. What is a transverse wave? In which medium—solid, liquid or gas—can it be produced?
A transverse wave is one in which particles vibrate perpendicular to the direction of propagation. It can be produced only in solids.
8. Define the term amplitude of a wave. Write its S.I. unit.
Amplitude is the maximum displacement of a particle from its mean position. S.I. unit: metre (m).
9. How is the frequency of a wave related to its time period?
Frequency is the reciprocal of time period: f = 1/T.
10. Arrange the speed of sound in gases (Vg), liquids (Vl) and solids (Vs) in ascending order.
Vg < Vl < Vs
11. State the speed of (i) light and (ii) sound in air.
(i) Speed of light ≈ 3 × 10⁸ m/s (ii) Speed of sound in air ≈ 330 m/s
12. Answer the following: (a) Can sound travel in vacuum? (b) How does the speed of sound differ in different media?
(a) No, sound cannot travel in vacuum. (b) Speed of sound is maximum in solids, less in liquids and least in gases.
13. Flash of lightning reaches us earlier than the sound of thunder. Explain the reason.
The speed of light is much greater than the speed of sound.
14. How does the speed of sound in air vary with temperature?
The speed of sound in air increases with increase in temperature.
15. Complete the following sentences: (a) Sound cannot travel through vacuum because it requires a ______. (b) When sound travels in a medium, the particles of the medium ______ but the disturbance ______. (c) A longitudinal wave is composed of compression and ______. (d) A transverse wave is composed of crest and ______. (e) Wave velocity = frequency × ______.
(a) medium (b) vibrate, travels (c) rarefaction (d) trough (e) wavelength
