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NCERT Solutions for Class 8, Science, Chapter 13 - Sound, , 1, Choose the correct answer., Sound can travel through, , a). gases only, , b). solids only, , ¢), liquids only, , d). solids, liquids and gases., Ans: (d) solids, liquids and gases., Sound can travel in any medium., , 2. Which of the following voices is likely to have minimum frequency?, , a). Baby girl, , b). Baby boy, , ¢). Aman, , d). A woman, , Ans: (c) A man, , Frequency of a sound is directly proportional to pitch. Here, the pitch of a man, is minimum., , 3. In the following statements, tick ‘T’ against those which are true, and ‘F*, against those which are false., , a). Sound cannot travel in vacuum., , Ans: T, , Vacuum is an enclosed place in which there are no molecules or matter., Therefore, sound cannot travel through vacuum where there is no molecule or, atom to induce vibrations., , b). The number of oscillations per second of a vibrating object is called its, time period., , Ans: F, , The number of oscillations per second of a vibrating object is known as its, , frequency.
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c). If the amplitude of vibration is large, sound is feeble., Ans: F, Sound is feeble for small amplitude., , d). For human ears, the audible range is 20 Hz to 20,000Hz., Ans: T, For human ears, the audible range is 20 Hz to 20,000Hz., , €). The lower the frequency of vibration, the higher is the pitch., , Ans: F, , The pitch of a sound is proportional to its frequency. As the frequency of, vibration increases, the pitch of the sound also increases and vice versa., , f), Unwanted or unpleasant sound is termed as music., Ans; F, Unwanted or unpleasant sounds are known as noise., , Se oe eee ee Oe, , If one is subjected to loud unpleasant sound continuously for a long time, then it, may cause temporary hearing impairment., , 4, Fill in the blanks with suitable words., a), Time taken by an object to complete one oscillation iscalled, Ans: Time taken by an object to complete one oscillation is called time period., , , , b). Loudness is determined by the of vibration., Ans: Loudness is determined by the amplitude of vibration., ¢). The unit of frequency is, , , , Ans: The unit of frequency is hertz (Hz),, , d), Unwanted sound is called ;, Ans: Unwanted sound is called noise,, , e). Shriliness of a sound is determined by the of vibration., Ans: Shriliness of a sound is determined by the frequency of vibration.
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5. A pendulum oscillates 40 times in 4seconds. Find its time period and, frequency., , Ans: Time period is defined as the time required to complete one oscillation., Frequency is defined as the number of oscillations per unit time., , Here,, , Time t= 4s, , Number of oscillations n= 40, , Frequency f =", , 40, Thus, f =—, 4, , “f =10 Hz, Time period is the inverse of frequency of an object., Therefore, 1-4, , 1, Ty, “T=0.1s, , 6. The sound from a mosquito is produced when it vibrates its wings at an, average rate of SOOvibrations per second. What is the time period of the, vibration?, , Ans: Time period is the inverse of frequency of an object., , Therefore, T=, , , , {.3, 500, -.T=0,002s, , 7. Identify the part which vibrates to produce sound in the following, instruments., , a). Dholak, , Ans: Dholak has stretched membranes called head that vibrate to produce sound., , b). Sitar, , Ans: In sitar, the stretched strings are plucked and when they vibrate, they, produce sound., , c). Flute
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Ans: In flute, the air column inside it vibrates when blown which produces, sound,, , 8. What is the difference between noise and music? Can music become noise, sometimes?, , Ans: Noise is an unpleasant sound which is a result of irregular vibrations. Music, is a sound which is pleasant to hear. Yes, music can sometimes become noise, , when it is played at very high volumes on loudspeakers., , 9, List sources of noise pollution in your surroundings., Ans: Following are the major sources of noise pollution:, , |, Sounds of vehicle horns., , li. Explosions that include bursting of crackers and explosives., iii, Sounds of machines and loudspeakers., , iv. Sounds of television and transistor radio at high volumes., v. Sounds of kitchen appliances,, , 10. Explain in what way noise pollution is harmful to humAns:, Ans: Noise pollution is harmful to humans as it may cause many health-related, Fr en VPN Ch, eee, , 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 buy? Explain your answer., , Ans: | would suggest that the house be three lanes away from the roadside, because the house next to the roadside will be more noisy compared to the house, three lanes away from the road., , 12. Sketch larynx and explain its function in your own words., , Ans: Larynx is a part of the throat. It moves when we swallow something. There, are two vocal cords inside the Larynx. The air passes through a small gap, between them. The lungs force the air into the space when we speak and this, vibrates the vocal cord, resulting in the production of sound. That is why larynx, is also known as sound box.
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Tap view of Lary, , 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?, , Ans; 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, because speed of light is greater than the speed of sound and therefore, light, travels faster than thunder
