Sound (NCERT Solutions, Class 8)

(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.

(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- (c) A man

(3)In the following statements, 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 objects 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)

Ans-

(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.

Ans-

(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 a sound is determined by the frequency of vibration.

(5) A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Ans-  Number of oscillations of pendulum = 40

Time taken in 40 oscillations = 4 sec

Time period = Time taken/Number of oscillations

= 4/40

= 1/10

= .1 sec

Frequency of pendulum = Number of oscillations/Time taken

= 40/ 4

= 10 Hz

Hence, time period for oscillation is .1 and frequency of pendulum is 10Hz.

(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- Number of vibrations = 500

Time taken in vibrations = 1 sec

Time period =Time taken/Number of oscillations

= 1/500

= .002

Hence, time period is .002 second.

(7) Identify the part which vibrates to produce sound in the following instruments.

(a) Dholak   (b) Sitar   (c) Flute

Ans-

(a) The membrane of dholak vibrates to produce sound.

(b) The wires of sitar vibrate to produce sound.

(c) The pipe of the flute vibrates to produce sound.

(8) What is the difference between noise and music? Can music become noise sometimes?

Ans-

Differences between noise and music are tabulated below:-

Noise Music
Sound produced by the vehicles, machines etc. Sound produced by the musical instruments.
These are unpleasant sound. These are pleasant sound.
Eg:- Sound of crackers, sound of vehicles etc. Eg:- Sound produced by musical instruments like guitar, sitar, tabla etc.

 

Normally, music is very pleasant sound. But, sometimes music can be noise if it produced unrhythmic sounds.

(9)List sources of noise pollution in your surroundings.

Ans- The sources of noise pollution in our surroundings are listed below:-

(i) Sounds of crackers

(ii) Sound of vehicles

(iii) Sound of machines

(iv) Sound of electrical appliances

(v) Sound of television etc.

(10) Explain in what way noise pollution is harmful to human.

Ans- Harms of the noise pollution

(i) Noise pollution caused lack of sleep.

(ii) Noise pollution caused hypertension (high blood pressure)

(iii) Noise pollution also caused anxiety.

(iv) Noise pollution also caused temporary or permanent hearing impairment.

(11) Your parents are going to buy a house. They have been offered one on the road side and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Ans– I would suggest house three lanes away from the roadside because it will receive less noise of vehicles of road in comparison of house on the road side.

(12) Sketch larynx and explain its functions in your own words.

Ans- Sketch of larynx is showing below:-

schoolhelpbygunjan.wordpress.com | Voice box | Diagram | Sound | NCERT | Class 8In humans, the sound is produced by the voice box or the larynx. It is the upper end of the windpipe. Two vocal cords are stretched across the voice box. These vocal cords leave a slit between them for the air through by lungs to pass from it. When air passes from the slit, cords vibrate and produce sound. The quality of voice depends upon the tight and loose of the cord.

(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 3 × 108m/s and speed of sound is 332 m/sec. Therefore, light travels at a very high speed in comparison of speed of sound. Hence, lightning is seen earlier and thunder is heard later.

 

Helping Topics

Sound

Practice sheet, class 8

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