NCERT Solutions Science for Class 8 Chapter 13 SOUND | Class8 Science Chapter 13

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NCERT Solutions Science for Class 8 Chapter 13 SOUND

NCERT Solutions Science for Class 8 Chapter 13 SOUND

science for class 8 Chapter 13 question answer

 Sound can travel through (a) gases only (b) solids only (c) liquids only (d) solids, liquids and gases.

Answer – (d) solids, liquids and gases.

 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 statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.

 (a) Sound cannot travel in vacuum. (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)

 (a) Sound cannot travel in vacuum. (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 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.
$$\displaystyle \text{Frequency}\,\,\text{=}\,\,\frac{{\text{No}\text{.}\,\,\text{of}\,\,\text{Oscillations}}}{{\text{Time taken to oscillate}}}$$
$$\displaystyle =\frac{{40}}{4}$$
Frequency = 10 hertz
$$\displaystyle \text{Time period}\,\,\text{=}\,\,\frac{\text{1}}{{\text{frequancy}}}$$
$$\displaystyle =\frac{{1}}{10}$$
Time period = 0.1 sec

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?
$$\displaystyle \text{Time period}\,\,\text{=}\,\,\frac{\text{1}}{{\text{frequancy}}}$$
$$\displaystyle =\frac{{1}}{500}$$
Time period = 0.002 sec

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

 (a) Dholak (b) Sitar (c) Flute

(a) Dholak – In this the stretched membrane vibrates.
(b) Sitar – In this the stretched string or string is vibrated.
(c) Flute – The air going inside the tube vibrates.

8. What is the difference between noise and music? Can music become noise sometimes?
Noise – Unpleasant sounds are called noise.
Music – Music is one which is pleasing to the ear.

Yes, music become noise sometimes if a music becomes too loud.

9. List sources of noise pollution in your surroundings.
Answer : The sources of noise pollution which are found in our surrounding environment are –

• Home Appliances Cooler, Mixer, Machine sound.
• the sound of the loudspeaker
• From the sound of the TV played at a loud voice.
• noises from bus, car vehicles

10. Explain in what way noise pollution is harmful to human.
Answer : Excessive or unwanted sounds in the environment are called noise pollution.
The presence of excessive noise in the surroundings can cause many health related problems. Insomnia, excessive stress (high blood pressure), anxiety and many other health disorders can be caused by noise pollution. The hearing ability of a person who is continuously under the influence of loud sound becomes temporarily or permanently lost.