NCERT Q&A for Class 8 Science Chapter 13 Sound
1. Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Soln:
Answer is (c) A man
Explanation:
When compared to the voices of a baby boy, baby girl, and woman, the voice of a man has a lower pitch. As the pitch of a man is low, which is proportional to the frequency of a sound, the man’s voice is of minimum frequency as compared with others.
2. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency
Soln:
The number of oscillations per second of the vibrating body is known as the frequency of oscillation.
Frequency = Total number of oscillations
Total time taken
= 50/5
= 10 Hz
Time period is the time taken to complete one oscillation. It is also the inverse of frequency.
Time period = 1/ Oscillating frequency
= 1/10
= 0.1 s
∴ frequency = 10 Hz
Time period = 0.1 s
3. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Soln:
Time period is defined as the time taken to complete one oscillation. It is also the inverse of frequency.
Time period = 1
Oscillation frequency
Oscillation frequency = 500 Hz
Time period = 1/500 = 0.002 s
4. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute
Soln:
a) Dholak- It consists of a head which is a stretched membrane. Vibrations are set into these stretched strings when the head is beaten gently, these vibrations produce sound and thus Dholak produces sound.
b) Sitar – It is a musical instrument. Stretched strings are part of it. Vibrations are produced when the string is plucked when played. These vibrations produce sound, thus sitar produces sound.
c) Flute – It’s an instrument which has holes in it. It is a hollow pipe. The air inside the pipe is set into vibration when air is blown over its mouth and this produces a pleasant sound.
5. What is the difference between noise and music? Can music become noise sometimes?
Soln:
Music is sound which is pleasant to hear. Sounds from flutes, pianos and violins are pleasant to hear.
Noise are sounds which are unpleasant to hear.
Sounds which are unpleasant to hear are:
(a)Sounds from bus horns and truck horns.
(b)Electrical generator sounds.
(c)Gunshot sounds.
(d)Jackhammer sounds
Yes, sometimes when the music is played at high volumes, it becomes noise.
6. List sources of noise pollution in your surroundings.
Soln:
Noise pollution sources are
(a) Bus and car horns.
(b) Firecrackers and loudspeakers.
(c) High volumes in televisions and transistors.
(d) mixers at home
e) Sirens from factories
7. Explain in what way noise pollution is harmful to human.
Soln:
A number of health issues are associated with noise pollution. They are as follows
(a) Stress
(b) Headache
(c) Hearing loss
(d) Insomnia
(e) Hypertension
8. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Soln:
It is better to buy the house which is three lanes away from the roadside as there will be less noise as compared to the one on the main road. The noises can be caused by vehicles. As the distance between the source and the listener increases, the intensity of noise decreases. So it’s better to buy the house which is three lanes away.
9. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Soln:
Speed of the light is more than the speed of sound. Thus, lightning is seen first which is accompanied by thunder later.