In this post we will see science class 8th chapter 13 solution by expert teacher.
You can check the Complete Science Solution here – NCERT Class 8th Science Solutions – Complete Solution List
Chapter 13 Solutions
1. Choose the correct answer.
1. Sound can travel through
(a) gases only (b) solids only
(c) liquids only (d) solids, liquids, and gases
Answer – Sound can travel through solids, liquids, and gases.
2. Voice of which of the following is likely to have minimum frequency?
(a) Baby girl (b) Baby boy
(c) A man (d) A women
Answer – A man
3. In the following statement, tick “T” against those which are true, and “F” against, those which are false.
(a) Sound cannot travel in vaccum. (T/F)
(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)
Answer – (a) Sound cannot travel in vaccum. (T)
(b) The number of oscillations per second of a vibrating object is called its time period. (F)
(c) If the amplitude of vibration is large, sound is feeble. (F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
(e) The lower the frequency of vibration, the higher is the pitch. (F)
(f) Unwanted or unpleasant sound is termed as music. (F)
(g) Noise pollution may cause partial hearing impairment. (T)
4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called ___________.
(b) Loudness is determined by the ___________ of vibration.
(c) The unit of frequency is __________.
(d) Unwanted sound is called ____________.
(e) Shrillness of a sound is determined by the __________ of vibration.
Answer – (a) Time taken by an object to complete one oscillation is called _time period .
(b) Loudness is determined by the _amplitude__ of vibration.
(c) The unit of frequency is hertz (Hz) .
(d) Unwanted sound is called noise.
(e) Shrillness of a sound is determined by the_frequency of vibration.
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer – Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period 
6. 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?
Answer – No of vibration in 1 second = 50
Time period 
Frequency of oscillations = 500 Hz
Time period 
7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak – Stretched membrane.
(b) Sitar – Stretched string.
(c) Flute – Air column.
8. What is the difference between noise and music? Can music become noise sometimes?
Answer – The unpleasant sound is called noise. Whereas pleasant sound is called music. Noise can produce so many health hazards, whereas noise music brings about something effect. Yes, music can become a noise sometimes when the musical instruments produce very high volume sounds.
9. List source if noise pollution in your surrounding.
Answer – The source of noise pollution are:
1. The sound produced by buses and trucks.
2. The sound produced at the construction site.
3. The sound produced by playing of T.V, radio and loudspeaker.
4. Bursting of cracker, and sound of big machines in the factories.
5. Home appliances such as mixer, desert cooler, etc.
10. Explain in what way noise pollution is harmful to human.
Answer – The noise pollution may cause many health related problems. Lacks of sleep, hypertension, anxiety, etc are some of the problems that may caused due to noise pollution. Moreover, a person who is exposed to a loud sound continuously may get temporary or permanent deafness.
11. 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 explain your answer.
Answer – I would suggest my parents to buy the house which is three lanes away from the roadside. This would protect us from noise pollution which is three lanes away from the roadside. This would protect us from noise pollution. Which is maximum at roadside building.
12. Sketch larynx and explain its function in your own words.
Answer – Larynx is a part of the throat. It is responsible for production of sound. A sketch of a human larynx is shown in the following figure.
Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.
13. 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?
Answer – The light travels at the speed of 3,00,000 km/s. Which is very large in comparison to the speed of sound which travels at the rate of 330 m/s in air. That is why lightning is seen earlier and thunder is heard later.
You can check the Complete Science Solution here – NCERT Class 8th Science Solutions – Complete Solution List
You can see other posts here,
- Class 8th NCERT Science Solution Chapter 12 – Friction
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- Class 8th NCERT Science Solution Chapter 7 – Conversation of Plant and Animal
- Class 8th NCERT Science Solution Chapter 6 – Combustion and Flame
- Class 8th NCERT Science Solution Chapter 5 – Coal and Petroleum
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