CBSE Class 8 Science Sound Solved Examples

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Solved Examples

Ex.1: Figure shows the mean position A and the extreme positions B and C of the bob of an oscillating pendulum. Identify which of the following distances represent a quarter oscillation and which represent half an oscillation
(a) BA (b) AC (c) BAC (d) CAB (e) AB (f) ABA

Sol.: BAC + CAB is a complete oscillation. Thus each of BAC and CAB is half an oscillation. Each of BA, AC and AB is half of BAC. So these three are quarter oscillation. ABA = AB + BA. As AB = BA = quarter oscillation, ABA is also half an oscillation.

Ex.2: The frequency of a source of sound is 100 Hz. How many time does it vibrate in a minute?
Sol.: Here, v = 100 Hz.
1 minute = 60 seconds
\ No of vibrations = 100 × 60 = 6000

Ex.3: Figure shows the displacement of a particle with distance in a periodic wave moving along a string.
(a) Find the wavelength (b) If the wave travels at a speed of 10 cm/s, find the time period and the frequency of the wave.

Sol.: (a) We see from the figure that the distance between consecutive crests is 1 cm. Hence, the wavelength is 1 cm.

Ex.5: An oil-tanker explodes at sea. At that instant an aircraft and a submarine were present vertically above and below the oil-tanker respectively, at equal distances from it. Find the ratio of the times taken by sound waves to reach them. (Speed of sound in air = 340 m/s, speed of sound in sea water = 1,520 m/s.)
Sol.: Let the aircraft and the submarine be x m above and below the tanker respectively Time taken by sound to reach the aircraft

Ex.6: A person clapped his hands near a cliff and heard the echo after 5 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 ms–1?
Sol.: Given, Speed of sound, v = 346 ms–1
Time taken for hearing the echo, t = 5 s
Distance travelled by the sound
= v × t = 346 ms–1 × 5 s = 1730 m
In 5 s sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person
= 1730 m/2 = 865 m.

Basic Level Assignment
1. What is sound and how is it produced?
2. Why is sound wave called a mechanical wave?
3. How does the sound produced by a vibrating object in a medium reach your ear?
4. Give two practical applications of reflection of sound waveS.

5. Distinguish between periodic motion and oscillatory motion. Is the motion of an electric fan periodic or oscillatory?
6. What do you understand by the equilibrium position of a pendulum?
7. Explain with a diagram what you understand by a complete oscillation?
8. When a wave propagates in a medium, what is transferred from one place to other, matter or energy?
9. Give three examples of transverse waves.
10. A longitudinal wave travels from east to west in air. In which direction do the particles of air move?
11. A sitar player plucks the wire of a sitar to a side and releases it. After a short time, each part of the wire starts vibrating in a direction perpendicular to the wire. Is the propagation of this disturbance a longitudinal or a transverse wave?
12. A long spring is fixed at one end. A person holding the other end compresses the spring with a jerk. The compression travels along the length of the spring. Is it an example of longitudinal wave or transverse wave?
13. Is sound wave longitudinal or transverse?
14. Name two quantities that vary periodically at a place in air as a sound wave travels through it.
15. In which of the three media, air, water or steel, does sound travels the fastest?
16. Which has larger frequency-infrasonic sound or ultrasonic sound?
17. What is reverberation? How can it be reduced?
18. What is loudness of sound? on What factors does it depend?
19. Explain how bats use ultrasound to catch a prey.
20. How is ultrasound used for cleaning?
21. Explain how defects in a metal block can be detected using ultrasound.
22. Explain how the human ear works.

Choose the correct option(s) in the following:

1. If a wave completes 20 vibrations in 2.5 sec, its frequency is
(A) 20 Hz (B) 8 Hz
(C) 200 Hz (D) 50 Hz

2. An observer standing at the sea coast observes 54 waves reaching the coast per minute. If the wavelength of wave is 10 m, the velocity of the wave will be
(A) 4.5 m/sec (B) 9 m/sec
(C) 18 m/sec (D) 27 m/sec

3. Sound waves cannot pass through
(A) a solid-liquid mixture (B) an ideal gas
(C) a liquid gas mixture (D) perfect vacuum

4. If five waves are produced in one second, then time period of one wave is
(A) 0.20s (B) 0.25 s
(C) 0.10 s (D) 0.05 s

5. The distance between two successive rarefactions is called
(A) frequency (B) wavelength
(C) wave velocity (D) amplitude

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