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: (d) 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: (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. (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)
Question: 4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called ....................
Answer: Time period
(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: Frequency and Amplitude
Question: 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer: Frequency is defined as the number of oscillations per second.
Since time period is time taken to complete one oscillation. It is the inverse of the frequency.
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: Since time period is time taken to complete one oscillation
Fig: Musical instruments
Question: 7. Identify the part which vibrates to produce sound in the following instruments:
Answer: In Dholak, there is a stretched membrane at the top known as head. When the head is beaten, the stretched membrane vibrates and thus the sound is produced.
Answer: In Sitar, there are many stretched strings. When these strings are plucked, the string vibrates and thus the sound is produced.
Answer: In Flute, there is hollow pipe. When air is blown into it, the air inside it starts vibrating and thus the sound is produced.
Question: 8. What is the difference between noise and music? Can music become noise sometimes?
Answer: 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.
Answer: Following is the list of sources of noise pollution in my surroundings:
(i) Horns of vehicles.
(ii) Bursting of crackers.
(v) Television and radio at high volumes.
(vi) Kitchen appliances like mixer, etc.
(vii) Air conditioners.
Fig: Sources of noise pollution
Question: 10. Explain in what way noise pollution is harmful to humans.
Answer: There are many harmful effects of noise pollution on humans like:
(i) Lack of sleep.
(ii) High blood pressure (Hypertension).
(iii) Temporary or permanent impairment 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: I will suggest my parent to buy a house which is away from the roadside, because house at roadside would be noisy because of running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter.
Question: 12. Sketch larynx and explain its function in your own words.
Answer: Larynx is that part of throat which is responsible for producing sound. Inside larynx, there are two vocal cords, which are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When we speak, air is forced into this small gap by the lungs which prompt vocal cords to vibrate. When the lungs force 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 sound is 330 m/s while that of light is 3 x 108 m/s. Thus, speed of light is much higher than the speed of sound. This is the reason light reaches us before sound during a lightning.