NCERT Solutions for Class 8 Science Chapter 11 - Sound

NCERT TEXTBOOK QUESTIONS SOLVED
 

Q. 1. Choose the correct answer. Sound can travel through:
 (a) gases only (b) solids only
 (c) liquids only (d) solids, liquids and gases.
 Ans. (d) solids, liquids and gases.
 

Q. 2. Voice of which of the following is likely to have minimum frequency?
 (a) Baby girl (b) Baby boy
 (c) A man (d) A woman.
 Ans. ( a) Baby girl.
 

Q. 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.
 (b) The number of oscillations per second of a vibrating object is called its time period.
 (c) If the amplitude of vibration is large, sound is feeble.
 (d) For human ears, the audible range is 20 Hz to 20,000 Hz.
 (e) The lower the frequency of vibration, the higher is the pitch.
 (f) Unwanted or unpleasant sound is termed as music.
 (g) Noise pollution may cause partial hearing impairment.
 Ans. (a) True
(b) False
(c) False
(d) True
(e) False
(f) False
(g) True.
 

Q. 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.

Ans. (a) time period
(b) amplitude
(c) hertz
(d) noise  
(e) frequency

Q. 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
 Ans. Number of oscillations = 40 Total time = 4 seconds Time taken to complete one oscillation = 

So time period = 0.1 second.

Q. 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?
 Ans. Total vibrations = 500 Time taken = 1 second Time taken to complete one vibration =   =  0.002 seconds.

Q. 7. Identify the part which vibrates to produce sound in the following instruments. (a) Dholak (b) Sitar (c) Flute.
 Ans. (a) Stretched membrane
(b) Strings
(c) Air column.
 

Q. 8. What is the difference between noise and music? Can music become noise sometimes?
 Ans. The sound which is unpleasant for our ears is called noise while music is the sound which is pleasant for our ears. Music becomes noise sometimes when it crosses the bearable range of sound for our ears.
 

Q. 9. List sources of noise pollution in your surroundings.
 Ans. Sources of noise pollution: Honking of horns, loudspeakers, loud sounds of machines in factories, loud sounds of T.V., radio, domestic appliances etc.
 

Q. 10. Explain in what way noise pollution is harmful to humans.
 Ans. Harmful effects of noise pollution:
(i) It causes deafness.
(ii) It causes mental illness.
(iii) It causes headache and high blood pressure.
 

Q . 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 to buy? Explain your answer.
 Ans. I would suggest my parents to buy the house three lanes away from the roadside. This house would safeguard our health and peace of mind.
 

Q. 12. Sketch larynx and explain its function in your own words. Ans. Larynx is also known as voice box. It has vocal cords which have air column vibrating in them, which cause sound in humans.

Q. 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. The speed of light is more than that of sound. Due to more speed the light reaches us before the sound does. So lightning is seen earlier and thunder is heard later.