NCERT solution for class 8 // Science // chapter 13 - Sound

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NCERT solution for class 8  // Science // chapter 13 - Sound

Chapter 13 - Sound 

Exercise 

Q1. Choose the correct answer 

1. Sound can travel through 

a) gases only     b ) Solid only     c) liquid only   d) solid liquid 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 woman  

3. In the following statement tick  T against those which are true and F against those which are false .

a) sound cannot travel in vacuum .

b) The number of oscillation per second of A vibrating object is called its time period.

c) If the amplitude of vibration is largest sound is feeble .

d) For human ears the audible range is 20 Hz to 20000 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 .

Answer- a)  true b)  false  c) false. d) false  e) false  f) true

Q4. Fill in the blanks with suitable words .

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 .

d) Unwanted sound is called noise.

e) Shrillness of sound is determined by the frequency of vibration .

Q5. A pendulum oscillates 20 times in 4 seconds find its time period and frequency .

Answer-  Given ,

number of oscillation = 40 . 

Total time - 4 seconds .

Time taken to complete one oscillation= 4/40 = 0.1 second 

Frequency = 1/ Time period = 1/ 0.1 = 10 Hz.  

Q6. 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- Given , 

Total vibration= 500  

time taken = 1 second  

Time taken to complete one vibration  = 1  / 500 = 0.002 second .

So time period = 0.002 second. 

Q7. Identify the part which vibrates to produce sound in the following instruments .

a) Dholak     b) sitar       c) flute  .

Answer- a) Streached membrane 

               b) Strings 

              c) Air column  

Q8. What is the difference between noise and music can music becomes noise sometimes.

Answer- Any unpleasant sound is called noise while music is the sound which is pleasant for ear . some time music becomes noise when it crosses the bearable rangers of sound for ears

Q9. List sources of noise pollution in your surroundings ? 

Answer-  Sources of  noise pollution are loudspeaker ,machinery sound in factories ,loud sound of TV, domestic appliances like, mixer grinder ,vehicle horn etc

Q10. Explain in what way noise pollution is harmful to human.

Answer- Effect of noise pollution are as follow 

a) Noise pollution can causes deafness.

b) It also causes headache and high blood pressure .

c) It causes mental illness .

Q11. 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?

Answer- I would suggest my parents to buy a house three lanes away from the roadside because it is safe for our health and away from the noise so also good for mental health

Q12. Sketch larynx and explain its function in your own words? 

Answer-  Function of a larynx ..  

Larynx is also called voice box . It has it two flapping muscles of vocal cord which have air column vibrating in them and which cause sound 

Q13. Lightning and thunder take place in the sky at the same time and at the same distance from US lightening is seen earlier and thunder is heard later can you explain why? .

Answer- Lightning and thunder both incident are causes at the same time but the speed of light is more than the speed of sound so lightening is seen earlier and thunder is heard later .


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