Class 8 physics Chapter - Sound, Reflection of sound CBSE and NCERT Study help
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Q.1 How does a sound making object differ from one that is silent?
Answer: Sound making object can vibrate easily.
Q.2 How does sound from a sound producing body travel through air to reach our ears?
Answer: Sound from a sound producing body travel through air to reach our ears by making regular compression and
rarefaction.
Q.3 Why are the voices of men, Women and children different?
Answer: The voices of men, women and children differ from each other due to the following factors:
(i) The pitch is one of the most essential factors to determine the voice of the individual. For example, men have lower-pitched voices while women have higher-pitched voices.
(ii) Second factor which brings in the difference in the voices is the length and the thickness of vocal cords.
Q.4 Why a sound cannot be heard on the moon?
Answer: A sound cannot be heard on the moon due to absence of medium air
Q.5 How do astronauts talk to one another on the surface of moon and Why?
Answer: Astronauts communicate with the help of radio waves on moon because these waves do not need a medium to travel
Q.6 If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track?
Answer: This is because sound can travel through railway track (solid ) faster than through air.
Q.7 State one observation from everyday life which shows that sound travels much more slowly than light?
Answer: We hear thunder after lightining.
Q.8 What is the scientific name for the following? The number of vibrations made per second.
Answer: Frequency
Q.9 Why do we not hear the screams of a bat?
Answer: This is because the screams are ultra sonic(above 20,000 hertz) which human ear cannot hear.
Q.10 Which of the following frequency of sound can be heard by a dog but not by a man? (a) 50,000 hertz (b) 15,000 hertz
Answer: A sound of frequency 50,000 hertz can be heard by a dog but not by a man
Q.11 Explain how, noise pollution {or excessive loud noise} is harmful to human beings.
Answer: Noise pollution can lead to a number of health-related problems. Some of them are as follows:
(i) Hearing loss (ii) inability to sleep (iii) Hypertension (iv) Severe headache (v) Stress
Q.12 State the various measures which can be taken to control {or reduce} noise pollution in our surroundings.
Answer: Noise pollution can be reduced by using silencers in vehicles, industrial machines, and home appliances reducing use of vehicle horns, running TV, radio and music systems at low volumes. Planting of trees along roads and buildings also help to reduce noise pollution.
Q.13 Give two causes of noise pollution from the homes.
Answer: . In the home noise pollution is caused by television radio and music systems at high volume, some kitchen appliances, desert coolers, air conditioners etc.
Q.14 Sound of different pitch can be produced using a flute. Explain, how?
Answer: If we blow a flute, the air column vibrates and produces sound. Flute contains small air columns, when air is bowed through the mouth piece the air columns start vibrating which produces sound.
Q.15.How does Sound produced by humans?
Answer: In humans sound is produced by the vioce box or larynx. It is the upper part of the wind pipe. Two vocal cords are stretched across the voice box leaving a narrow slit. When the lungs force air through the slit, it vibrates and produces sound.
Q. 16. How Jal Tarang produces sound?
Answer: In Jal-tarang musical instrument the cup containing minimum water produces the sound of lowest frequency or lowest pitch. As the amount of water in the cup goes on increasing, the frequency (or the pitch) of the sound produced also goes on increasing.
Q. 17. If 125 oscillations are produced in 5 seconds, what is the frequency in hertz?
Answer: in 5 seconds 125 oscillations are produced
So, in 51 seconds 125/5 = 25 oscillations are produced Hence, the frequency = 25Hz
Q. 18. What is the frequency of a vibrating body whose time-period is 0.05 second?
Answer: f = 1/t = 1/0.05 = 100/5 = 20HZ
Q. 18. State true or false
(i) Sound can be produced by beating an object irrespective of whether there are vibrations or not.
(ii)The sound of a tabla normally has a lower pitch than the sound of a violin.
(ii)A short flute will produce sound of lower pitch than a long flute.
Answer:
(i) –False [Sound is produced from a vibrating object.]
(ii) -True [The sound of a tabla normally has a lower pitch than the sound of a violin.]
(iii) –True[A short flute will produce sound of lower pitch than a long flute.]
Q.19. Give reasons Why Trees planted along the roads reduce noise?
Answer: Trees are planted along the roads and around buildings so that it could cut off sound vibrations produced from vehicles speakers etc. by absorbing them, hence leading in prevention of Noise pollution.
Q.20. what are transverse and longitudinal waves?
Answer: Longitudinal waves: A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called longitudinal wave. Example: sound waves.
Transverse waves: A wave in which the particles of the medium vibrate at right angles to the direction, in which the wave is moving, is called transverse wave. Example: Light waves.
Q.21. Explain how bats use ultrasounds to catch a prey ?
Ans: Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves. The high pitched ultrasonic squeaks of the bat are reflected from the obstacles or prey and returned to bat’s ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like.
Q.22. Explain how moths of certain families are able to escape capture from a bat ?
Ans: Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know where a bat is flying nearby and are able to escape capture.
Q. 23.How does the ear drum of human ear vibrate ?
When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates.
Q. What is the role of hammer bone, anvil bone, stirrup bone and cochlea of human ear in hearing a sound
Ans: The vibrations due to the ear drum is amplified several times by three bones in the middle ear. The pressure variations in the inner ear turned in to electrical signals by the cochlea.
Q.24. How does the stethoscope help the doctors in listening to the sound of the patients heart beat?
Ans. In a stethoscope, the sound produced within the body of a patient is picked up by a sensitive diaphragm and then reaches the doctor's ear by multiple reflections. Q.3. Explain how defects in a metal block can be detected using ultrasound. Ans. To detect minor cracks or flaws in metal block, ultrasonic waves are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in metal block, these waves get reflected back
Q.25. How is ultrasound used for cleaning?
Ans. The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with cleaning solution. As the ultrasonic vibrator is switched on, high frequency vibrations are set up and the dust, grease and dust particles get detached and the object gets thoroughly cleaned.
Answer: Sound making object can vibrate easily.
Q.2 How does sound from a sound producing body travel through air to reach our ears?
Answer: Sound from a sound producing body travel through air to reach our ears by making regular compression and
rarefaction.
Q.3 Why are the voices of men, Women and children different?
Answer: The voices of men, women and children differ from each other due to the following factors:
(i) The pitch is one of the most essential factors to determine the voice of the individual. For example, men have lower-pitched voices while women have higher-pitched voices.
(ii) Second factor which brings in the difference in the voices is the length and the thickness of vocal cords.
Q.4 Why a sound cannot be heard on the moon?
Answer: A sound cannot be heard on the moon due to absence of medium air
Q.5 How do astronauts talk to one another on the surface of moon and Why?
Answer: Astronauts communicate with the help of radio waves on moon because these waves do not need a medium to travel
Q.6 If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track?
Answer: This is because sound can travel through railway track (solid ) faster than through air.
Q.7 State one observation from everyday life which shows that sound travels much more slowly than light?
Answer: We hear thunder after lightining.
Q.8 What is the scientific name for the following? The number of vibrations made per second.
Answer: Frequency
Q.9 Why do we not hear the screams of a bat?
Answer: This is because the screams are ultra sonic(above 20,000 hertz) which human ear cannot hear.
Q.10 Which of the following frequency of sound can be heard by a dog but not by a man? (a) 50,000 hertz (b) 15,000 hertz
Answer: A sound of frequency 50,000 hertz can be heard by a dog but not by a man
Q.11 Explain how, noise pollution {or excessive loud noise} is harmful to human beings.
Answer: Noise pollution can lead to a number of health-related problems. Some of them are as follows:
(i) Hearing loss (ii) inability to sleep (iii) Hypertension (iv) Severe headache (v) Stress
Q.12 State the various measures which can be taken to control {or reduce} noise pollution in our surroundings.
Answer: Noise pollution can be reduced by using silencers in vehicles, industrial machines, and home appliances reducing use of vehicle horns, running TV, radio and music systems at low volumes. Planting of trees along roads and buildings also help to reduce noise pollution.
Q.13 Give two causes of noise pollution from the homes.
Answer: . In the home noise pollution is caused by television radio and music systems at high volume, some kitchen appliances, desert coolers, air conditioners etc.
Q.14 Sound of different pitch can be produced using a flute. Explain, how?
Answer: If we blow a flute, the air column vibrates and produces sound. Flute contains small air columns, when air is bowed through the mouth piece the air columns start vibrating which produces sound.
Q.15.How does Sound produced by humans?
Answer: In humans sound is produced by the vioce box or larynx. It is the upper part of the wind pipe. Two vocal cords are stretched across the voice box leaving a narrow slit. When the lungs force air through the slit, it vibrates and produces sound.
Q. 16. How Jal Tarang produces sound?
Answer: In Jal-tarang musical instrument the cup containing minimum water produces the sound of lowest frequency or lowest pitch. As the amount of water in the cup goes on increasing, the frequency (or the pitch) of the sound produced also goes on increasing.
Q. 17. If 125 oscillations are produced in 5 seconds, what is the frequency in hertz?
Answer: in 5 seconds 125 oscillations are produced
So, in 51 seconds 125/5 = 25 oscillations are produced Hence, the frequency = 25Hz
Q. 18. What is the frequency of a vibrating body whose time-period is 0.05 second?
Answer: f = 1/t = 1/0.05 = 100/5 = 20HZ
Q. 18. State true or false
(i) Sound can be produced by beating an object irrespective of whether there are vibrations or not.
(ii)The sound of a tabla normally has a lower pitch than the sound of a violin.
(ii)A short flute will produce sound of lower pitch than a long flute.
Answer:
(i) –False [Sound is produced from a vibrating object.]
(ii) -True [The sound of a tabla normally has a lower pitch than the sound of a violin.]
(iii) –True[A short flute will produce sound of lower pitch than a long flute.]
Q.19. Give reasons Why Trees planted along the roads reduce noise?
Answer: Trees are planted along the roads and around buildings so that it could cut off sound vibrations produced from vehicles speakers etc. by absorbing them, hence leading in prevention of Noise pollution.
Q.20. what are transverse and longitudinal waves?
Answer: Longitudinal waves: A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called longitudinal wave. Example: sound waves.
Transverse waves: A wave in which the particles of the medium vibrate at right angles to the direction, in which the wave is moving, is called transverse wave. Example: Light waves.
Q.21. Explain how bats use ultrasounds to catch a prey ?
Ans: Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves. The high pitched ultrasonic squeaks of the bat are reflected from the obstacles or prey and returned to bat’s ear. The nature of reflection tells the bat where the obstacle or prey is and what it is like.
Q.22. Explain how moths of certain families are able to escape capture from a bat ?
Ans: Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know where a bat is flying nearby and are able to escape capture.
Q. 23.How does the ear drum of human ear vibrate ?
When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates.
Q. What is the role of hammer bone, anvil bone, stirrup bone and cochlea of human ear in hearing a sound
Ans: The vibrations due to the ear drum is amplified several times by three bones in the middle ear. The pressure variations in the inner ear turned in to electrical signals by the cochlea.
Q.24. How does the stethoscope help the doctors in listening to the sound of the patients heart beat?
Ans. In a stethoscope, the sound produced within the body of a patient is picked up by a sensitive diaphragm and then reaches the doctor's ear by multiple reflections. Q.3. Explain how defects in a metal block can be detected using ultrasound. Ans. To detect minor cracks or flaws in metal block, ultrasonic waves are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in metal block, these waves get reflected back
Q.25. How is ultrasound used for cleaning?
Ans. The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with cleaning solution. As the ultrasonic vibrator is switched on, high frequency vibrations are set up and the dust, grease and dust particles get detached and the object gets thoroughly cleaned.
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