ADDITIONAL QUESTIONS AND ANSWERS :
NUMERICALS :
1. The human heart beats 75 times in one minute. Calculate its period and frequency.
Ans: Number of vibrations =75, time = 1 minute = 60 seconds.
Period T = time taken for one vibration = 60/75 = 0.8 seconds
Frequency = number of vibrations in one second 75/60 = 1.25 Hz.
2. If the period of a wave is 0.005 seconds, calculate its frequency.
Ans: Period T = 0.005 seconds
Frequency ϑ = 1/T = 1/0.005 = 200 Hz.
3. A country boat anchored near the coast is rocked by waves 30 times per minute. The distance between two consecutive crests of the wave is 8 metres. Calculate the speed of the wave.
Ans: The frequency of the wave = 30 times per minute = 30/60 = 0.5 Hz
The wavelength λ of the wave = 8 metres
∴ Speed of the wave = ϑλ = 0.5 x 8 = 4 m/s.
4. A child hears the echo of a whistle from a cliff 4 seconds after it was blown. Calculate the distance of the cliff from the child. Take velocity of sound as 340 m /s.
Ans: Let the distance of the cliff from the child be ‘d.’ Therefore the distance travelled by the sound from the child to the cliff and back to the child is 2 d.
The echo is heard after 4 seconds.
The distance travelled by sound= 4 x 340=1360 m
Therefore 2 d = 1360 or d = 680 metres.
∴ The cliff is 680 metres away from the child.
5. A gun is fired from a distance of 688 m from a mountain. After what time of firing will the echo be heard. The speed of sound in air is 344 m/s.
Ans: Given speed of sound= 344 m/s,
Distance of mountain from the gun = 688 m
The distance to be travelled by the sound to hear an echo is = 1376 m
∴ The time taken to travel 1376 m = 1376 / 344 = 4 s.
6. A sonar echo from an underwater whale takes 2.6 seconds to return to the detector on ship. How far is the whale from the ship. Velocity of sound in sea water is 1530 m/s.
Ans: Let the distance of the whale from the ship be‘d’.
Therefore the distance travelled by the sound is 2 d.
The echo returns after 2.6 seconds.
The distance travelled by sound (2 d)-2.6 x 1530= 3978 m
∴ d = 1989 metres.
The whale is 1989 metres away from the ship.
7. The sound produced by a source is heard at a distance of 680 m after 2 s. If the frequency of sound is 1.36 kHz, calculate the wavelength of the sound wave.
Ans: Speed of sound wave =680 / 2 = 340 m/s
Speed = wavelength x frequency
⇒ 340= λ x 1360 or λ = 340/ 1360 = 25 cm.
8. Ultrasound sent from a ship is reflected from an object and the same is received by the detector after 5 seconds. Calculate the depth of the object under water. The speed of sound in seawater is 1530 m/s.
Ans: Let the depth of the object below the sea level be’d’. Therefore the distance travelled by the ultrasound is 2 d = the speed
of ultrasound x time;
v = s / t
2 d = 1530 x 5 = 7650m
⇒ d = 7650/2 = 3825 m.
9. The wavelength of a sound wave is 20 cm and its frequency is 5 kHz. Find:
(a) The speed of the sound wave.
Ans: Given: wavelength λ= 0.2 m = 20 cm; frequency v = 5 kHz = 5000 Hz
Speed = λv = 0.2 x 5000 = 1000 m/s.
(b) The time taken to travel a distance of 2 km.
Ans: t = s / v
2 Km = 2000 m will be 2000 / 1000 = 2 s.
10. The frequency of a sound wave is 200 Hz and its wave length is 50 cm. Calculate:
a. The speed of the wave.
Ans: v = vλ =200 x 0.5 = 100 m/s.
b. The time taken by the wave to travel a distance of 1 km.
Ans: v = s / t or t = s / v⇒ 1000 /100 = 10 seconds.
A. Name the following:
1. The unit of frequency.
Ans: Hertz
2. Any two characteristics of a sound wave.
Ans:(i) frequency (ii) amplitude
3. Sound of single frequency.
Ans: Tone
4. The region of (a) high pressure (b) low pressure in a sound wave.
Ans: (a) Compression (b) Rarefaction
5. Phenomenon in which the same sound is heard again and again after a short time.
Ans: Echo
6. Any two instruments working on the principle of multiple reflection of sound.
Ans: Megaphone, stethoscope.
7. Two sound absorbing materials.
Ans: Felt and fibre board
8. The three bones of the middle ear.
Ans: Hammer, Anvil and Stirrup
9. Any two animals which produce infrasound.
Ans: Whales and elephants
10. Any two animals which produce ultrasound.
Ans: Dolphins, bats and tortoises
11. The technique of getting images of various parts of the heart using ultrasound.
Ans: Echo Cardio Graphy.( E.C.G)
B. Give one term for the following:
1. Rapid to and fro motion of an object.
Ans: Vibration.
2. The matter or substance through which sound is transmitted.
Ans: Medium of propagation
3. A disturbance that moves.
Ans: Wave
4. Sound of single frequency.
Ans: Tone
5. Persistence of sound by repeated reflection
Ans: Reverberation
6. Number of oscillations per second
Ans: Frequency
7. A sound which is a mixture of several frequencies.
Ans: Note
C. Answer in short:
1. The factor on which pitch of sound depends.
Ans: The pitch of sound depends upon the frequency
2. The Sl units of i) Wavelength (ii) Frequency
Ans:(i) Metres (ii) Hertz
3. The factor on which loudness of sound depends.
Ans: The loudness of sound depends on amplitude.
4. The factors on which the speed of sound depends
Ans: (i) The nature of the medium
(ii) The temperature of the medium
(iii) The pressure of the medium.
5. An example each of a (i) transverse wave and (ii) longitudinal wave
Ans: (i) Light waves are transverse waves
(ii) Sound waves are longitudinal waves.
6. The duration for which the sensation of sound persists in our brain.
Ans: Sensation of sound persists in our brain for 0.1 second or for 1/10 of a second.
7. The relationship between speed ‘v’, wavelength ‘λ ‘ and period T
Ans: v= λ/T
8. The relationship between speed ‘v’, wavelength ‘λ ‘and frequency ϑ.
Ans: v = λϑ
9. Any common observation which shows that light travels faster than sound.
Ans: (i) The flash of lightning is seen before thunder is heard.
(ii) The flash of firecrackers burning at a distance is seen before their sound is heard.
10. The principle involved in the working of whispering galleries.
Ans: Multiple reflection of sound waves.
11. State the characteristic of sound involved in each of the following:
(i). Anthony identifies his friend from his voice.
(ii). A person can distinguish the sound of a man from that of a woman.
Ans: (i)Timbre. (ii) Pitch
E. State the specific function of each of the following parts of the ear
1. Pinna
Ans: Collects the sound waves.
2. Ear drum
Ans: Vibrates when sound waves fall on it.
3. The three tiny bones of the middle ear.
Ans: Amplify the vibrations
4. Cochlea.
Ans: Converts the vibrations into electrical signals
5. Auditory nerve
Ans: Carries electrical signals to the brain.
F. Define the following :
1. Wavelength of a longitudinal wave.
Ans: Wavelength is the distance between two consecutive compressions or rare factions.
2. Frequency.
Ans: Frequency is the number of compressions or rarefactions that cross a given point in a unit time
3. Amplitude
Ans: Amplitude is the magnitude of maximum disturbance in the medium on either side of the mean value
4. Intensity of sound.
Ans: Intensity of sound is the amount of sound passing through a unit area in one second
5. A longitudinal wave.
Ans: A wave in which the direction of motion of the particles is parallel to the direction of motion of the wave is called a longitudinal wave.
6. Transverse wave.
Ans: A wave in which particles oscillate in a direction perpendicular to the direction of the wave is called a transverse wave. Light is an example for transverse wave.
7. Pitch.
Ans: Pitch is the characteristic of sound by which two sounds having the same loudness can be distinguished.
8. Echo.
Ans: Echo is the hearing of the same sound again after some time.
G. Answer the following:
1. How is an echo heard.
Ans: The sensation of sound persists in our brain for 1/10th of a second. If the reflected sound waves reach our ears after 1/10th of a second, the same sound is heard again or an echo is heard.
2. An echo is not heard in a hall of length less than 17.2 m. Why.
Ans: The sensation of sound persists in our brain for one-tenth of a second. To hear an echo the reflected sound waves should reach our ears after one-tenth of a second. In one-tenth of a second sound travels a distance of 34.4 m. Therefore the reflecting surface should be at a distance of 17.2 m or the hall should not be of length less than 17.2 m.
3. How does a tone differ from a note.
Ans: A tone is a sound of single frequency, while a note is a mixture of several frequencies.
4. A shehnai has a tube attached to a conical opening. Why.
Ans: A shehnai has a tube attached to a conical opening so that the sound that enters the tube is prevented from spreading by successive reflections and it is directed towards the audience.
5. What is reverberation.
Ans: Reverberation is the persistence of sound in big halls due to repeated reflection of sound.
6. Why is excessive reverberation undesired.
Ans: If the reverberation is too long i.e. excessive, the sound becomes blurred,distorted and confusing. Therefore, such type of reverberation is undesirable.
7. Sound does not travel through a vacuum. Why.
Ans: Sound waves are mechanical waves which propagate due to the oscillation of the particles of the medium. A vacuum is an empty space without particles and hence the sound waves cannot travel through a vacuum.
8. Explosions in space, though sufficiently loud, cannot be heard on earth. Give a reason.
Ans: Sound requires a material medium for its propagation. The explosions that take place in space are the events occurring in a vacuum. So they are not heard on earth, though sufficiently loud.
9. The sound of a distant train approaching a station cannot be heard by standing on the platform. But the same can be heard it we put our ear to the railway line. Why.
Ans: The velocity of sound in steel is more than 15 times that in air. Therefore sound travels faster in steel than in air. Hence the sound of the train can be heard through the rails
10. The flash of lightning and thunder are produced simultaneously.But thunder is heard after the flash is seen. Give reason.
Ans: Light travels faster than sound. Though a flash of lightning and thunder are produced simultaneously, the light of the flash reaches a person first because light travels faster. Therefore, the flash of lightning is seen first followed by thunder.
11. On the moon, one astronaut cannot hear the other speaking. Give reason.
Ans: Sound requires a medium for its propagation. The space surrounding the moon is empty. i.e. a vacuum. In the absence of a medium one astronaut cannot hear the other speaking.
12. When a ringing bell is touched it stops producing sound. Why.
Ans: A bell produces sound as long as it vibrates. When it is touched, the vibrations are stopped. Therefore, it stops producing sound.
13. What is ultrasound or ultrasonic sound.
Ans: The sound of frequency above 20 kHz (20,000 Hz) is called ultrasound.
14. What is echocardiography.
Ans: The technique of obtaining images of the various parts of the heart by using ultrasound is called echocardiography.
15. What is ultrasonography.
Ans: The technique of obtaining images of internal organs or a foetus using ultrasonic waves is called ultrasonography.1.
16. Why is ultrasound used in ultrasonography.
Ans: Ultrasound is used in ultrasonography because it has a high frequency and is able to travel along well defined paths even in the presence of obstacles.
17. Sound in audible range cannot be used to detect cracks or flaws in a metallic block. Give reason.
Ans: Sound in audible range or ordinary sound cannot be used to detect cracks or flaws in metallic blocks because they have longer wavelength and they bend at the corners of the location of a defect.
18. When is sound said to be infrasonic or infrasound.
Ans: When the frequency of sound is below 20 Hz to which the human beings are insensitive.
19. Some animals get disturbed before an earthquake. Why.
Ans: Earthquakes produce infrasound of low frequency before the main shock waves. Some animals which are sensitive to such infrasound are alerted and therefore they are disturbed.
20. Why does a school bell produce sound when struck with a hammer.
Ans: When a hammer strikes the school bell, it vibrates. Vibrations produce compressions and rarefactions which are spread in all directions.Therefore the bell produces sound.
21. When an object is hit with a greater force, it produces louder sound.Why.
Ans: When an object is hit with a greater force, the object vibrates with greater force and produces waves with greater energy or greater amplitude. Therefore the sound is louder.
22. In order to produce louder sound the string of a guitar is to be pulled hard. Why.
Ans: The loudness of sound depends on the amplitude of vibration. When the string of a guitar is pulled harder, the string vibrates with a greater amplitude and therefore produces a louder sound.
23. Why does a louder sound travel greater distances.
Ans: A louder sound has greater amplitude and therefore greater energy. Therefore a louder sound can travel greater distances due to its high energy content.
24. We have to speak loudly if a person standing at a distance has to hear our voice. Why.
Ans: A loud sound has high energy. Such sounds can travel longer distances because of its high energy content. Therefore when we speak loudly the sound travels a greater distance and persons standing at a distance can hear it.
25. The loudness of sound decreases as one moves away from the source producing the sound. Why.
Ans: The amplitude of a sound wave decreases as the wave travels.Therefore its loudness also decreases as one moves away from the source producing the sound.
26. Why are the floors of big halls and auditoriums covered with a carpet.
Ans: In big halls and auditoriums there is excessive reverberation which is unwanted. Carpets are used to absorb sound and minimise reverberation.
27. Why do concert halls, conference halls, cinema halls, do not have straight ceilings but curved ones. Why.
Ans: When the ceilings are curved, they reflect sound. The reflected sound reaches all corners of the hall and people can hear it clearly.
28. In large halls a concave board is placed on the stage behind the speaker. Why.
Ans: A concave board placed behind the speaker is called a sound board reflects the sound and spreads it evenly across the width of the hall so that all the people in the hall can hear the speaker clearly
H. Give two points to distinguish between:
1. Compression and rarefaction
Ans:
|
Compression
|
Rarefaction |
| (i) Compression is a region of high particle density. | i) Rarefaction is a region of low particle density. |
| (ii) The pressure is low in this region. | (ii) The pressure is high in this region. |
2. Sound wave and light wave
Ans:
|
Light wave
|
Sound wave
|
| Light wave is a transverse wave. | Sound wave is a longitudinal |
| It does not require a medium for its propagation. |
It requires a medium for its propagation.
|
| It can travel through a vacuum. | It cannot travel through vacuum. |
| It is a mechanical wave. | It is not a mechanical wave. |
3. Longitudinal wave and transverse wave
Ans:
|
Longitudinal wave
|
Transverse wave
|
| The direction of motion of the particles is the same as the direction of the wave. | The direction of motion of the is perpendicular to the direction of the wave |
| It requires a medium for its propagation. | A medium is not always necessary. |
|
It travels as compressions and rarefactions
|
It travels as troughs and crests.
|
4. Infrasound and ultrasound
Ans:
|
Infrasound It has frequency below 20 Hz |
Ultrasound It has frequency above 20 kHz. |
| It has low penetrating power | It has high penetrating power |