SCERT KERALA TEXTBOOKS SOLUTIONS & STUDY NOTES: Class 9 Physics (English Medium) Chapter 07 Wave Motion
SCERT Solutions for Class 9 Physics Chapter wise
Class 9 Study Notes Physics - Chapter 7: Wave Motion
Wave motion - Study Notes, Practice Questions and Answers
I. Study Notes
* Wave motion.
Wave motion is the propagation of disturbances, produced in one part of a medium to the other parts by the vibration of its particles. Water wave, light wave, sound wave etc are examples of wave motion. All waves do not need a medium for propagation. The waves which need a medium for propagation are a mechanical wave. Water waves and sound waves are mechanical waves.
Mechanical waves are classified into two, namely transverse waves and longitudinal waves.
* Transverse waves:-
* Characteristics of waves
1. Amplitude(a): The maximum displacement of a particle from its mean position is amplitude. It is denoted by the letter 'a'.
2. Period(T): It is the time taken for a particle to make one complete
vibration.
3. Frequency(f): Frequency is the number of vibrations in one second.
Frequency, f = n/t where n – number of vibrations and t is the time
taken for n vibrations.
It is also equal to, f = 1/T. The unit of frequency is hertz (Hz).
4. Wavelength (λ): Wavelength is the distance advanced by the wave by the time a particle completes one vibration. Its unit is metre.
Speed of the wave, v = fλ. Unit of speed is m/s.
* Crests and Troughs.
The distance from one crest to the adjacent crest or the distance from one trough to the adjacent trough is the wavelength of the transverse wave.
* Relation between wavelength and frequency.
For a wave of constant speed, frequency and wavelength are inversely proportional.
* Longitudinal Wave
rarefactions (R) in the medium are responsible for the propagation of a longitudinal wave through a medium.
The distance between two successive compressions or two successive rarefactions is the wavelength of a longitudinal wave. The propagation of sound through air is pictured below. The letters 'C' & 'R' indicate the compressions and rarefactions respectively.
* Speed of Sound:
Sound travels through solid, liquid and gaseous medium. But the speed is different in different media. It is through solid, sound travels with the greatest speed. And it is least in gases. The speed of sound also depends on
temperature. The speed of sound increases with the temperature of the medium.
* Reflection of Sound.
Sound reflects like a light when hits on surfaces. The rate of reflection is greater from smooth surfaces.
* Multiple reflections of sound.
Sound getting reflected repeatedly from different objects is multiple reflections.
Multiple reflections of sound are made use of in Megaphone, Hone, Stethoscope, musical instruments like trumpets, shehnai etc.
* Reverberation.
The booming of sound due to multiple reflections of sound from various objects in a hall is called Reverberation.
The persistence of audibility is responsible for reverberation.
The sensation of hearing produced by a sound is retained for a period of 0.1 s. These characteristics of the ear are called persistence of audibility.
* Echo.
Echo is the phenomenon of hearing a sound by reflection from a surface after hearing the original sound. Echo is felt only if the distance from the source to the reflecting surface is 17 m.
* Acoustics of building.
It is the branch of science that deals with the conditions to be fulfilled in the construction of a building for clear audibility. Giving curvature to the ceiling, using soundboard, making rough the walls, using carpets and curtains etc are methods being employed in this respect.
* Ultrasonic Sound.
The audible limit of human beings is from 20 Hz to 20000 Hz. The sound of frequency greater than 20000Hz is called Ultrasonic sound.
* Uses of Ultrasonic sound.
i. It is used to clean spiral tubes, machine parts etc than cannot be cleaned directly using articles like a brush. Object to be cleaned is dipped in a suitable solution and Ultrasonic waves are passed through the solution. The highly vibrating solution detaches grease, dust etc from the object.
ii. It is used to detect cracks and flaws in large metal blocks.
iii. Ultrasonic waves are used in Echocardiography and Ultrasonography which are the tools for diagnosis and treatment.
iv. Ultrasonic sound is used in SONAR which is used to measure the depth
of the ocean. The Ultrasonic waves that are sent from the water surface gets reflected from the bottom of the sea. The distance from the surface of the water to the bottom bed of the sea can be calculated from the time of travel.
then, 2h = vt Or h = vt/2
* Seismic Waves.
Seismic waves form as a result of Earth's Quakes and volcanic eruptions. They originate from the epic centre of the earthquake. Seismology is the study of seismic waves. It is in the Richter scale, the intensity of the Earth Quake is measured.
Seismic waves are classified into three; namely Primary Waves (P waves),
Secondary waves (S waves) and Surface waves.
Of these waves, the Primary wave is the fastest wave.
There are two kinds of surface waves. They are Rayleigh waves and Love waves.
Surface waves are the reason for major damages caused by Earthquake. The amplitude of the waves obtained using a seismograph determines the intensity of an Earthquake.
A tsunami is series of Gigantic waves that form in water as a result of underwater Earthquakes. It causes large scale destructions in coastal regions. DART is the system that gives a warning about Tsunami.
Practice Questions & Answers.
1. The period of a wave is 0.2 s. Find its frequency.
Answer: frequency f = 1/T = 1/0.2 = 10/2 = 5 Hz
2. Some features of a wave are given.
i. have Compressions and Rarefactions.
ii. particles of the medium vibrate parallel to the direction of the wave.
a. Identify the wave. b. Give one example of such a wave.
Answer: a. Longitudinal Wave. b. Sound.
3. Calculate the speed of the wave of frequency 10Hz and wavelength 2m.
Answer: Speed of the wave, v = fλ = 10x2 = 20m/s
4. It is depicted the propagation of a wave through a medium.
a. What kind of wave is this?
b. What do C& R represent in the picture?
c. The distance from one C to the next C is ...... of the wave.
(amplitude/frequency/wavelength)
Answer:
a. Longitudinal wave.
b. C – Compression. R- Rarefaction.
c. Wavelength.
5. Write down the two major differences between Longitudinal Wave and the Transverse wave.
Answer:
i) Transverse wave.
* Particles of the medium vibrate perpendicular to the direction of the wave.
* Have Crest and Troughs.
ii) Longitudinal Wave
* Particles of the medium vibrates parallel to the direction of the wave.
* Have Compressions and Rarefactions.
6. The wave shown in the figure travels 24 m in 0.5 s.
(Compressions/ Rarefactions/Crests/Troughs)
b. Locate two pints that are in the same state of vibration as that of C.
c. Find three points in the mean position which are in the same state of vibration.
d. Find the wavelength and amplitude of the wave.
e. Calculate the frequency of the wave.
f. What is the speed of the wave?
Answer:
a.Crests.
b. G&K .
c. B,F&J OR D,H&L
d. Wavelength = 8m Amplitude = 2 cm
e. Frequency, f = n/t = 3/0.5 = 30/5 = 6Hz.
f. Speed of the wave, v = fλ = 6x8 = 48 m/s
7. The rate of reflection at rough surface is ....... (more/less)
Answer: less.
8. What feature of sound is made use of in the stethoscope?
Answer: Multiple reflections.
9. Give a few examples for the devices in which multiple reflections of sound is made use.
Answer: Stethoscope, hone, megaphone, trumpet, soundboard.
10. What is reverberation?
Answer: Reverberation is the persistence of sound as a result of multiple reflections.
11. A sound reaches our ears retains there for a short interval of time.
a. What is called these characteristics of the ear? b. How long does it retain?
Answer: a. Persistence of vision. b. 1/10 s
12. Echo is the phenomenon of hearing a sound after hearing the original sound.
a. Which phenomenon of sound is responsible for this?
b.What should be the minimum distance between the listener and the reflecting surface if sound travels through air?
c. What will be the change in this distance if sound travels through water?
Answer:
a. Reflection of sound.
b. 17m
c. Greater than 17m.
13. What is the acoustics of a building?
Answer: It is the branch of science that deals with the conditions to be fulfilled in the construction of a building for clear audibility.
14. What all the things can we do in halls for getting clear audibility?
Answer: Makes the wall rough, use carpets on the floor, use folded curtains on the walls.
15. What is the audible limit of human beings?
Answer: From 20 Hz to 20000Hz.
16. What is known as ultrasonic sound?
Answer: Sound with a frequency greater than 20000Hz is called ultrasonic sound.
17. What is called the sound having a frequency above 20000Hz?
Answer: Ultrasonic sound.
18. Ultrasonic sound can be used to take the image of a heart. What is called this?
Answer: Echocardiography.
19. What is SONAR? Identify the wave used in this device.
Answer: SONAR is a device used to measure the depth of the ocean. Ultrasonic waves are used in this device.
20. Give a few uses of ultrasonic wave.
Answer: It is used in SONAR, Echocardiography, ultrasonography. It is also used to detect cracks and flaws in large metal blocks.
21. Give two examples for the utilisation of ultrasonic wave in the medical field.
Answer: Echocardiography and Ultrasonography.
22. Ultrasonic waves from a ship hits a rock at the bottom of the sea and comes back after 0.4s. Calculate the distance to the rock from the ship. The speed of sound through seawater is 1520 m/s.
Answer: time taken for to and fro journey, t = 0.4 s
Speed of the wave,v = 1520 m/s
total distance travelled by the wave, s = vt = 1520x0.4 = 608 m
Distance to the rock from the ship = s/2 = 608/2 = 304 m
23. The after-effects of Earthquake, Volcano etc in one region causes disaster in other regions also.
a. Name the waves responsible for this.
b. What is used to measure the intensity of this wave?
Answer:
a. Seismic waves.
b. Richter scale.
24. The study of seismic wave is known as seismology.
a. Which are the various seismic waves?
b. Of these which has the highest speed?
Answer:
a. Primary waves (P – waves), Secondary waves (S – Waves ), Surface waves (L – waves ).
b. Primary waves.
25. Two kinds of surface waves formed at the time of the Earthquake are responsible for the major disaster. Name the two waves.
Answer: Rayleigh waves and Love waves.
26. Limit of audibility to human beings is 20Hz to 20000Hz. If so what will be the limit of a wavelength of sound waves that are audible to human beings? (speed of sound in air = 340 m/s)
Answer: λ1 = 340/20000 = 0.017 m λ2 = 340/20 = 17 m
That is, the sound waves having wavelengths from 0.017m to 17 m is will be audible to human beings.
27. Wavelength of a sound having a frequency 2kHz is 35 cm. How much time will it take to travel 100 m?
Answer: Speed of the wave,v = fλ = 2x1000x0.35 = 700m/s
Time required to travel 1500m , t = s/v = 1500/700 = 2.1 s
28. The speed of a sound wave is 340 m/s and its wavelength is 34m. Is it audible to human beings? Justify your answer.
Answer: Frequency of the wave, f = v/λ = 340/34 = 10 Hz.
Since the frequency is less than 20Hz (less than the audible limit) it is inaudible to human beings.
29. Calculate the frequency of the wave having a wavelength of 1.5 km and speed of 340 m/s.
Answer: Frequency of the wave, f = v/λ = 340/1500 = 0.23 Hz.
30. In auditoriums, curved soundboards are placed behind the screen. What is the need for this?
Answer: It helps to reach sound everywhere in the hall by the multiple reflections of sound.
31. Fill the second pair according to the first.
a. sound: longitudinal wave; waves on the water surface: ..........
b. Transverse wave: Particles are vibrated perpendicular to the direction of a wave; Longitudinal wave: ……
c. amplitude: metre; frequency: …….
d. Stethoscope: Multiple reflections; Sonar: …….
Answer:
a. Transverse wave
b. Particles are vibrated parallel to the direction of wave
c. hertz. d. reflection.
32. A tuning fork of 256 Hz excited and pressed against a table surface.
a) what is the frequency of vibration of the table?
b) what is this phenomenon called?
Answer:
a) 256 Hz.
b) Induced vibration
33. Which wave is propagated from the epicentre of an Earthquake? Which instrument is used to measure the intensity of an Earthquake?
Answer: Seismic waves, Seismograph.
34. Sound travels 1020m through a medium in 3 seconds. Which could be the media among the following? Justify your answer.
(Aluminium, Steel, Air, Seawater)
Answer:
Speed of sound v = 1020/30 = 340 m/s. So Sound is travelled through the air. The speed of sound through all other mediums are very high.
35. The maximum displacement of a particle in a medium from its mean position is known as .............
Answer: Amplitude
36. The waves which need a medium for propagation are .....................
(light waves, Electromagnetic waves, mechanical waves, Radio waves)
Answer: mechanical waves
37. Bhooming of sound is heard when the sound is produced in an empty hall. What is this phenomenon called?
Answer: Reverberation
38. The instruments shown below are working by making use of some sound phenomenon.
b) The ceiling of certain halls are constructed with a curvature. What is its advantage?
Answer:
a) Multiple reflections.
b) To avoid spreading sound all over the hall due to multiple reflections.
39. Tie one end of a rope to a window. Wind a ribbon on the rope. Hold the other end of the rope and move it up and down. The speed of the wave is constant.
Tie one end of a rope to a window. Wind a ribbon on the rope. Hold the other end of the rope and move it up and down. The speed of the wave is constant.
b) The ribbon completes 5 vibrations in a second. Then what is the velocity of the wave through the rope?
c) If the vibration of the rope is doubled what change will happen to the length between A and B? Explain.
Answer:
a) Transverse wave
b) V = fxƛ
v = 5 x 0.20 =1m/s
c) The distance will be halved.
40. Observe the representation of waves formed on the surface of the water. (Speed of waves in the water is taken as 1500m/s)
b) If the diameter of the container is 90cm, what is the frequency of the wave?
c) If the same sound passes with a speed of 2500m/s through water after adding some salt, what change will happen to its wavelength?
Observe the representation of waves formed on the surface of the water. (Speed of waves in the water is taken as 1500m/s)
a) Which wave is formed on the surface of the water?
b) If the diameter of the container is 90cm, what is the frequency of the wave?
c) If the same sound passes with a speed of 2500m/s through water after adding some salt, what change will happen to its wavelength?
Observe the representation of waves formed on the surface of the water. (Speed of waves in the water is taken as 1500m/s)
a) Which wave is formed on the surface of the water?
b) If the diameter of the container is 90cm, what is the frequency of the wave?
c) If the same sound passes with a speed of 2500m/s through water after adding some salt, what change will happen to its wavelength?
Answer:
a) Transverse/ Mechanical
b) one and half cycle in 90cm. So ƛ =60cm
c) The wavelength ƛ=V/f = 2500/2500 =1m
The wavelength will increase
41. Answer the questions
a) Write two problems raising by multiple reflections of sound in auditoriums
b) Write two methods to solve this.
Answer:
a) Echo, Reverberation
b) Positioning of Window/Door, Use of a curtain, Make the floor rough, Adjust the distance of the hall
42. Fill up the blanks suitably
Sound waves in the air: Longitudinal form
Waves on the surface of the water: .................................
Answer:
Transverse wave
43. Diagrams of some musical instruments are given
b) Explain this phenomenon.
Answer:
a) Forced vibration
b) Forced vibration is the phenomenon of one object in vibration forcing another object to vibrate in contact with it.
44. The waves which are responsible for the earthquake originate from the epic centre of the earthquake.
a. Write down the name of these waves.
b. Name the instrument which is used to measure the intensity of the earthquake
Answer:
a. Seismic waves
b. Seismograph
45. Solve the problems given below.
a) Calculate the velocity of a wave with a frequency of 256Hz and wavelength 1.5m.
b) What change will occur to the wavelength of the above wave, if the frequency is increased to 512Hz without changing the medium?
c) Where the wavelength will be more when a sound wave from a tuning fork of frequency 256Hz travels through air and steel? Explain
(speed of sound: In steel – 5941m/s, In Air – 343m/s)
Answer:
a) V = fxƛ
256x1.5 = 384m/s
b) ƛ = v/f
= 384/512 = 0.75m
c) Wavelength is more when sound is travelling through steel
Wavelength when travelling through steel = v/f =5941/256 = 23.2m
Wavelength when travelling through air =v/f = 343/256= 1.3m
46. Graphic representation of a sound wave reaching the ears of a boy sitting at a distance buzzer source is given below.
b. What is the frequency of the wave if its speed through air 350m/s?
c. How long it will take for the sound to reach the ears of the boy if he is sitting 100m away from the buzzer source?
d. What changes for the medium can be suggested to make the boy hear the same sound in less time from the same distance.
Answer:
a. Particles are vibrating parallel to the propagation of sound.
b. v = f x ƛ
f = 350/5 = 70Hz (
c. Speed = distance/time
Time = 100/350
t = 0.285s
d. Any change in the Density, Humidity, Wind, Temperature will affect the speed.
47. Give a scientific explanation for the life situations below by making use of the hints given.
a) Glass window panes will break at times when thunder strikes .why?
(Hints. Natural frequency, Amplitude of vibration, Resonance, Forced vibration)
b) Some special arrangements are made to hear the sound clearly inside large halls.
(Hints: Carpets, Acoustics of buildings, Reflection of sound, Reverberation)
Answer:
a. Window panes are subjected to forced vibration during thunder. Some times the natural frequency of thunder will match the natural frequency of window panes. It will result in resonance and the window panes will start vibrating with higher amplitude. Naturally, the window panes will break into pieces.
b. The clarity of sound can be lost due to reverberation and echo due to the reflection of sound inside large halls. It can be effectively controlled by using carpets and curtains inside the hall. The science of the study of sound is the acoustics of buildings.
(The concepts must be used suitably and must be arranged properly)
48. The sound produced by a tuning fork of 480Hz travels with a speed of 360m/s through the air. Then
a. What is the wavelength of the wave through the air?
b. If the length of the air column through which the sound travelled is 3m, How many compressions will be there?
Answer:
a. ƛ =v/f = 360/480 = 0.75m
b. ƛ = 0.75m. Therefore in 3m 3/0.75 = 4.
so 4 compressions
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