1. How does the sound produced by a vibrating object in a medium reach your ear?
Ans:
When an object vibrates, it causes the surrounding particles of the medium (such as air) to vibrate. These vibrations create a series of compressions and rarefactions that travel through the medium. The sound is transferred from one particle to the next until it reaches your ear, where it is detected by the eardrum.
2. Explain how sound is produced by your school bell.
Ans:
When the school bell is struck with a hammer, it starts vibrating. These vibrations create compressions (regions of high pressure) and rarefactions (regions of low pressure) in the surrounding air. These sound waves travel through the air, and we hear the sound of the bell.
3. In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.
(a) Sound cannot travel in a 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 the vibration is large, the 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 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 a vacuum. (T)
Explanation: Sound requires a medium (solid, liquid, or gas) to travel. It cannot propagate in a vacuum.
(b) The number of oscillations per second of a vibrating object is called its time period. (F)
Explanation: The number of oscillations per second is called frequency, not time period. The time period is the time taken to complete one oscillation.
(c) If the amplitude of the vibration is large, the sound is feeble. (F)
Explanation: The loudness of sound is proportional to the square of the amplitude. A large amplitude produces loud sound, while a small amplitude produces feeble sound.
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
Explanation: Human ears can hear sounds in the frequency range of 20 Hz to 20,000 Hz. Sounds outside this range are inaudible.
(e) The lower the frequency of vibration, the higher the pitch. (F)
Explanation: Pitch is proportional to the frequency. A higher frequency produces a higher pitch, while a lower frequency produces a lower pitch.
(f) Unwanted or unpleasant sound is termed as music. (F)
Explanation: Unwanted or unpleasant sound is termed as noise, while music refers to melodious and pleasant sounds.
(g) Noise pollution may cause partial hearing impairment. (T)
Explanation: Prolonged exposure to noise pollution can lead to temporary or partial hearing impairment and other health issues.
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) The shrillness of a sound is determined by the __________ of vibration.
Ans:
(a) time period.
(b) amplitude .
(c) hertz.
(d) noise.
(e) frequency
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Ans:
The frequency of oscillation is the number of oscillations per second.
Frequency = Total number of oscillations ÷ Total time taken
= 40 ÷ 4
= 10 Hz
The time period is the time taken to complete one oscillation and is the inverse of frequency.
Time period = 1 ÷ Frequency
= 1 ÷ 10
= 0.1 s
∴ Frequency = 10 Hz
Time period = 0.1 s
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:
The time period is defined as the time taken to complete one oscillation and is the inverse of frequency.
Time period = 1 ÷ Oscillation frequency
Frequency = 500 Hz
Time period = 1 ÷ 500 = 0.002 s
∴ Time period of the vibration = 0.002 s
7. Identify the part which vibrates to produce sound in the following instruments.
Ans:
(a) Dholak – The stretched membrane (head) vibrates when it is struck, producing sound.
(b) Sitar – The stretched strings vibrate when plucked, producing sound.
(c) Flute – The air column inside the hollow pipe vibrates when air is blown over its mouth, producing sound.
8. What is the difference between noise and music? Can music become noise sometimes?
Ans:
Music: A sound that is pleasant to hear, such as sounds from instruments like flutes, pianos, and violins.
Noise: A sound that is unpleasant or disturbing to hear, such as sounds from bus horns, truck horns, generators, or jackhammers.
Can music become noise?
Yes, music can become noise when played at very high volumes or in an unpleasant manner.
9. List sources of noise pollution in your surroundings.
Ans:
Sources of noise pollution include:
(a) Vehicle horns (buses and cars)
(b) Firecrackers and loudspeakers
(c) Televisions and music systems at high volumes
(d) Kitchen mixers and grinders
(e) Sirens from factories and ambulances
10. Explain in what way noise pollution is harmful to humans.
Ans:
Noise pollution can cause several health issues, such as:
(a) Stress and anxiety
(b) Headaches
(c) Partial or complete hearing loss
(d) Insomnia (difficulty sleeping)
(e) High blood pressure (hypertension)
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.
Ans:
I would suggest buying the house that is three lanes away from the roadside.
Reason:
The house farther from the roadside will experience less noise pollution caused by vehicles. As the distance from the noise source increases, the intensity of the noise decreases, ensuring a quieter and more peaceful environment.
12. Sketch the larynx and explain its function in your own words.
When we swallow something, the larynx (or voice box) moves. Inside the larynx, there are two vocal cords. When we speak, air from the lungs passes through a small gap between these vocal cords. As the air flows through this gap, the vocal cords vibrate, and this vibration produces sound. The pitch and tone of the sound can change depending on how the vocal cords are stretched or relaxed, allowing us to make different sounds.
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:
Lightning and thunder occur simultaneously but are perceived differently because of the difference in their speeds.
Lightning is a form of light, and light travels much faster than sound.
Thunder is the sound produced by the lightning strike and travels much slower than light.
Since light travels faster, we see the flash of lightning first. The sound of thunder travels more slowly, so we hear it later. The time delay between seeing lightning and hearing thunder depends on the distance between us and the lightning strike. The farther the lightning, the greater the time delay between seeing the flash and hearing the thunder.