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NCERT Solutions for Class 8 Science chapter 13 – Sound

Back Exercise

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

Answers
(d) solids, liquids and gases

Question 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

Answers
(c) A man

Question 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.

Answers
(a) True
(b) False
(c) False
(d) True
(e) False
(f) False
(g) True

Question 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.

Answers
(a) time period
(b) amplitude
(c) hertz (Hz)
(d) noise
(e) frequency

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

Answers
The number of oscillations per second of the vibrating body is known as the frequency of oscillation.

Frequency = Total number of oscillations

Total time taken

= 50/5

= 10 Hz

Time period is the time taken to complete one oscillation. It is also the inverse of frequency.

Time period = 1/ Oscillating frequency

= 1/10

= 0.1 s

∴ frequency = 10 Hz

Time period = 0.1 s

Question 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?

Answers
Time period = 1

Number of vibrations per second = 500

Time period = Time/Number of vibrations

                    = 1/500

                    = 0.002sec

Question 7:
Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute

Answers
(a) Dholak - Dholak has a stretched membrane which vibrates to produce sound.

(b) Sitar – Stretched strings are part which vibrates to produce sound in sitar.

(c) Flute – Flute is an musical instrument which uses air column to produce sound.

Question 8:
What is the difference between noise and music? Can music become noise sometimes?

Answers
Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music.
Music can become noise at many instances. When someone tries to enjoy very loud music, it can be noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Question 9:
List sources of noise pollution in your surroundings.

Answers
Noise pollution sources in our surroundings are
(i) Televisions and transistors running at high volumes.
(ii) Loudspeakers and crackers.
(iii) Horns of buses, cars and trucks.
(iv) Home appliances such as mixer, desert cooler, etc.

Question 10:
Explain in what way noise pollution is harmful to human.

Answers
Noise pollution can lead to a number of health-related problems. Some of them are as follows:
(i) Hearing loss
(ii) Insomnia; inability to sleep
(iii) Hypertension
(iv) headache
(v) Stress

Question 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.

Answers
It is better to buy the house which is three lanes away from the roadside as there will be less noise as compared to the one on the main road. The noises can be caused by vehicles. As the distance between the source and the listener increases, the intensity of noise decreases. So it’s better to buy the house which is three lanes away.

Question 12:
Sketch larynx and explain its function in your own words.

Answers
Larynx is a part of the throat. It is responsible for production of sound. Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.

Question 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?

Answers
As light travels faster than sound in air, we can see lightening and then hear the thunder sound. Although both of them occur at the same time, yet the speed of light in air is 3*10^8 metre per second whereas the speed of sound in air is only 330 meter per second. So, by the time, the sound reaches us, we can already see lightening happen. Now, since they happen at far away places, it taken time for them to travel.