Sound
Class 8th Science CBSE Solution
- Choose the correct answer. Sound can travel through (a) gases only (b) solids only (c)…
- Which of the following voices is likely to have minimum frequency? (a) Baby girl (b) Baby…
- In the following statements, tick T against those which are true, and F against those…
- Fill in the blanks with suitable words. (a) Time taken by an object to complete one…
- A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.…
- The sound from a mosquito is produced when it vibrates its wings at an average rate of 500…
- Identify the part which vibrates to produce sound in the following instruments. (a) Dholak…
- What is the difference between noise and music? Can music become noise sometimes?…
- List sources of noise pollution in your surroundings.
- Explain in what way noise pollution is harmful to humans.
- Your parents are going to buy a house. They have been offered one on the roadside and…
- Sketch larynx and explain its function in your own words.
- Lightning and thunder take place in the sky at the same time and at the same distance from…
Exercises
Question 1.Choose the correct answer.
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases.
Answer:
The option (d) is correct i.e. solids, liquids and gases.
Sound can travel through any medium i.e. Solids, Liquids and Gases
Question 2.
Which of the following voices is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Answer:
The correct option is (c) i.e. A man
The voice of man has the least frequency.
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. (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)
Answer:
(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.
Answer:
(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 the vibration.
Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
The Frequency is the number of oscillations of a vibrating object per second.
Therefore, frequency (f) is = 
Time period is the time required to complete one oscillation. It is the inverse of time period. Therefore, time period (T) = 
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?
Answer:
Time Period given by the inverse of the frequency. Therefore,
Therefore,
Hence, the tome period of the vibration is 0.0002 seconds.
Question 7.
Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak (b) Sitar (c) Flute
Answer:
Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Answer:
The Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music.
Music can become noise at many instances. Music is of many types and everybody may not like each type of music. When someone listens very loud music, it can be noise for someone else.
Question 9.
List sources of noise pollution in your surroundings.
Answer:
The sources of noise pollution are as follows:
1) Televisions running at high volumes.
2) Use of Loudspeakers in religious or other functions.
3) Use of crackers.
4) Horns of buses, cars and trucks
Question 10.
Explain in what way noise pollution is harmful to humans.
Answer:
Noise pollution has adverse effects physical as well as mental behavior. The Constant exposure to noise pollution can create many health related problems like hypertension, insomnia, mental disorder and may even lead to loss of hearing.
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.
Answer:
There will be more noise in the house which is along the roadside due to noise produced by transportation vehicles. But, the house which is three lanes away from the roadside would have comparatively less noise. Therefore, it is better to take the house that is three lanes away from the roadside.
Question 12.
Sketch larynx and explain its function in your own words.
Answer:
In humans, the sound is produced by the box, which is called voice box or the larynx. Larynx is a part of the throat. It is responsible for production of sound. It has two vocal cord which are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the air. When the lungs force the air through the slit, the vocal cords vibrate, producing sound.
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?
Answer:
The speed of light is more than the speed of sound. The speed of light is 
whereas, the speed of sound is 343 m/s. Due to this, light reaches to us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.