Please refer to Sound Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams
Sound Class 9 Science Notes and Questions
The below Class 9 Sound notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook.
Refer to Chapter 12 Sound Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.
1. Production of Sound Sound is produced due to the vibration of objects. Vibration is the rapid to and fro motion of an object.
Vibrating objects are the source of all sounds Irregular, chaotic vibrations produce noise Regular, controlled vibration can produce music. All sound is a combination of pure frequencies. A stretched rubber band when plucked vibrates and produces sound.
2. Propagation of Sound When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its equilibrium position.
The disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.
A wave is a disturbance which moves through a medium by the vibration of the particles of the medium. So sound is considered as a wave. Sound waves Require medium for transmission.
Sound waves are called mechanical waves When a vibrating object moves forward, it pushes and compresses the air in front of it forming aregion of high pressure called compression (C). When the vibrating object moves backward, it forms a region of low pressure called rarefaction(R).
A vibrating object producing a series of compressions (C) and rarefaction (R)
In these waves the particles move back and forth parallel to the direction of propagation of the disturbance. Such waves are called longitudinal waves.
There is another kind of waves called transverse waves. In these waves the particles oscillate up and down perpendicular to the propagation of the direction of disturbance.
Sound propagates in a medium as a series of compressions (C) and rare factions (R).
Compressions are the regions of high pressure and density where the particles are crowded andare represented by the upper portion of the curve called crest.
Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough
Characteristics of a sound wave
Frequency of sound wave
The number of oscillations per unit time is called the frequency of the sound wave. It is represented by the symbol v (Greek letter nu). Its SI unit is hertz (Hz)
Time period of sound wave:
The time taken by wave to complete one oscillation is called time-period.
Frequency and time are represented as follows :-
V for one oscillation
T=1VorV=1TT=1VorV=1T
Amplitude of sound wave
The amplitude of sound wave is the height of the crest or tough.
It is represented by the letter A.
The SI unit is the same as that of density or pressure.
The wavelength is the distance between the “crests” of two waves that are next to each other.
The amplitude is how high the crests are.
Pitch and loudness of sound
The pitch of sound (shrillness or flatness) depends on the frequency of vibration.
If the frequency is high, the sound has high pitch and if the frequency is low, the sound has low pitch
Speed of sound
The speed of sound is more in solids, less in liquids and least in gases.
The speed of sound also depends on the temperature of the medium. If the temperature of the medium is more, the speed of sound is more
3. Reflection of Sound
Sound gets reflected at the surface of a solid or liquid and follows the laws of reflection.
- The angle of incidence is equal to the angle of reflection.
- The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane.
4. Echo
If we shout or clap near a reflecting surface like tall building or a mountain, we hear the same sound again. This sound which we hear is called echo. It is caused due to the reflection of sound. To hear an echo clearly, the time interval between the original sound and the echo must be at least 0.1 s.
Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.I s = 344
m/s x 0.1 s =34.4 mm/s x 0.1 s =34.4 m. So to hear an echo clearly, the minimum distance of the reflecting surface should be half this distance that is 17.2 m.
Reverberation
Echoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation.
In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by sound absorbing materials like compressed fibre boards, rough plaster or draperies.
5. Uses of Multiple Reflection Of Sound
i) Megaphones, horns, musical instruments like trumpets, etc. are deigned to send sound by multiple reflection in a particular direction without spreading in all directions.
ii) Doctors listen to sounds from the human body through a stethoscope. The sound of heartbeat reaches the doctor’s ears by multiple reflection.
iii) Generally the ceilings of cinema halls and auditoriums are curved so that sound after multiple reflection reaches all parts of the hall. Sometimes a curved sound board is placed behind the stage so that sound after multiple reflection spreads evenly across the hall.
6. Range of Hearing
Human beings can hear sound frequencies between 20 Hz and 2000 Hz., called as audible range of sound.
Sound whose frequency is less than 20 Hz is called infrasonic sound
Sound whose frequency is more than 2000 Hz is called ultrasonic sound
7. Uses of ultrasonic sound
Ultrasonic sound is used to clean objects like electronic Components, used to detect cracks in metal blocks, used in ultra sound scanners for getting images of internal organs of the human body used to break small stones formed in the kidneys into fine grains.
8. Sonar It is a device which uses ultrasonic waves to measure distance, direction and speed of underwater objects. The distance of the object can be calculated by knowing the speed of sound in water and the time taken between the transmission and reception of ultrasound
9.Structure of the human ear
The sound waves passes through the ear canal to a thin membrane called eardrum. The eardrum vibrates. The vibrations are amplified by the three bones of the middle ear calledhammer, anvil and stirrup. Middle ear then transmits the sound waves to the inner ear. The brain then interprets the signals as sound.
Questions for Class 9 Science Chapter 12 Sound
Question. A sonar echo takes 2.2 s to return from a whale. How far away is the whale?
Ans : Total time taken by the signal = 2.2 s
So, time taken the signal to reach the whale = 1.1 s
Distance of the whale = d (assume)
Speed of sound in sea water at 25°C = 1533 ms–1
So, distance of the whale,
d = Speed of the signal × Time taken
or d = 1533 m s–1 × 1.1 s = 1686.3 m
Question. What do you mean by a wave?
Ans : A wave is a vibratory disturbance in a medium which carries energy from one point to another without being a direct contact between the two points.
Question. What does wave transfer-matter or energy?
Ans : Energy.
Question. Where is density of air higher at compression or at rarefaction?
Ans : At compression.
Question. A gun is fired in the air at a distance of 660 m, from a person. He hears the sound of the gun after 2 s. What is the speed of sound?
Ans : Distance travelled by sound = 660 m,
Time taken by the sound = 2 s,
Speed of sound in air = ?
So, Speed of sound = Distance/Time
=660/2s=330m/s
Thus, the speed of sound in the air is 330 m/s.
Question. Guess which sound has a higher pitch-guitar or car horn?
Ans : Guitar has a higher pitch because it has higher frequency.
Question. What is intensity of sound?
Ans : The amount of sound energy passing through unit area each second is called the intensity of sound.
Question. Why are longitudinal waves called pressure waves?
Ans : Sound waves travels in the form of compression and rarefactions, which involve change in pressure, and volume of the air. Thus, they are called pressure waves.
Question. Sound travels faster on a rainy day than on a dry day. Why?
Ans :Sound travels faster on rainy day because the velocity of sound increases with increase in humidity. On rainy day, humidity is more thus, velocity of sound is also more.
Question. How moths of certain families are able to escape capture?
Ans : Moths of certain families can hear high frequency sounds (squeaks) of bat as they have sensitive hearing equipment. Thus, they get to know when a bat is near by and hence, able to escape its capture.
Question. What is SONAR?
Ans : SONAR (Sound Navigation And Ranging) is a technique for determining water depth and locating underwater objects, such as reefs, submarines and schools of fish.
Question. Define one hertz.
Ans : One hertz is one vibration per second.
Question. Define wavelength.
Ans : It is the distance between two nearest points in a wave which are in the same phase of vibration.
Question. What is the audible range of the average human ear?
Ans : An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.
Question. What is sound and how is it produced?
Ans : Sound is mechanical energy which produces a sensation of hearing. When an object is set into vibrations, sound is produced.
Question. Why is sound wave called as longitudinal wave?
Ans : Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave.
Question. Flash and thunder are produced simultaneously. But, thunder is heard a few seconds after the flash is seen, why?
Ans : The speed of light is 3 × 108 ms–1 whereas that of sound is 344 ms–1 in air. Thus, flash of lightning is seen at once, but sound takes few seconds to reach our ears.
Question. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Ans : No. of vibrations produced in 1 s = 100
No. of vibrations produced in 60 (sec) = 1 min =
100 × 60 = 6000.
Question. Name the two types of mechanical waves.
Ans : The two types of mechanical waves are :
(i) Transverse wave and
(ii) Longitudinal wave.
Question. Name the term associated with the travelling disturbance in a medium.
Ans : Wave.
Question. Do waves transport energy?
Ans : Yes.
Question. Do waves transport matter?
Ans : No.
Question. Sound is produced due to a vibratory motion, then why a vibrating pendulum does not produce sound?
Ans : The frequency of the vibrating pendulum does not lie within the audible range (20 Hz to 20,000 Hz) and hence, it does not produce audible sound.
Question. Why do echoes produced in an empty auditorium usually decrease when it is full of audience?
Ans : When the hall is empty there are no obstacles in between to reflect the sound other than the walls.
When the hall is full of audiences, the sound produced undergoes multiple reflections from the people and so it overlaps with the sound produced. Hence, the listener is not able to distinguish between the original sound and the echo.
Question. What is a wave?
Ans : A wave is a disturbance that travels in a medium due to repeated periodic motion of particles about their mean position – such that the disturbance is handed over from one particle to the other without the actual motion of the medium.
Question. What is a transverse wave?
Ans : It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave.
Question. What is a longitudinal wave?
Ans : It is a wave in which the particles of the medium vibrate in the direction of propagation of the wave.
Question. What do you understand by the term ultrasonic vibrations?
Ans : Sounds of frequency higher than 20,000 Hz are called the ultrasonics.
Question. What do you understand by the term echo?
Ans : The sound heard after reflection from a rigid obstacle is called an echo.
Question. Do the particles of the medium move from one place to another in a medium?
Ans : No.
Question. Does the velocity of wave motion depend on the nature of the medium?
Ans : Yes.
Question. Does the velocity of wave motion depend on the nature or motion of the source?
Ans : No.
Question. What are transverse waves? Give two examples.
Ans : A wave in which the particles of the medium vibrate up and down at right angle to the direction in which the wave is moving.
Example : (i) The waves produced by moving one end of a long spring up and down rapidly.
(ii) Ripples formed on the surface of water in a pond.
Question. What are crests and troughs of a wave?
Ans : The elevation in a transverse wave is called crest. It is that part of transverse wave which s above the line of zero disturbance of the medium. The depression in a transverse wave is called trough. It is that part of the transverse wave which is below the line of zero disturbances.
Question. A Sitarist tries to adjust the tension and pluck the string suitably, before playing the orchestra in a musical concert. By doing so what is he adjusting?
Ans : He is adjusting frequency if the sitar string with the frequency of the other musical instrument.
Question. If the tension in the wire is increased four times, how will the velocity of wave in a string varies?
Ans : Velocity of the wave in string is directly proportional to the square root of the tension thus if tension is increased 4 times, the velocity will be doubled.
Question. Explain, how is the principle of echo used by the dolphin to locate small fish as its prey?
Ans : Dolphins are aquatic animals which send out ultrasonic sound to communicate with each other. They have a sound sensing system which enables them to find animals underwater with great accuracy due to the echo of the ultrasonic sound produced by them.
Question. Give two practical applications of the reflection of sound waves.
Ans : (i) In stethoscope the sound of patient’s heartbeat reaches the doctor’s ears by multiple reflections in the tubes.
(ii) Megaphones are designed to send sound waves in particular direction are based on the reflection of sound
Question. What is the other name of a long flexible spring?
Ans : Slinky is the other name of a long flexible spring.
Question. Distinguish between tone and note.
Ans : A pitch is a particular frequency of sound, for example : 440 Hz.
A note is a named pitch. For example : Western music generally refers to the 440 Hz pitch as A, specifically A4.
Question. How do you account for the fact that two strings can be used to give notes of the same pitch and loudness but of different quality?
Ans : The ‘quality’ of a given note is determined by the overall effect of the harmonics present in it. The harmonics are multiples of the fundamental or basic frequency of the ‘note’. Depending on the conditions under which vibrations are taking place, sometimes we get one set of harmonics and sometimes another set.
The quality of the two notes will, therefore, different even though their fundamental frequencies may be the same.
Question. Can you produce both types of waves (i.e., longitudinal and transverse) on a slinky?
Ans : Yes, we can produce both types of waves (i.e., longitudinal and transverse) on a slinky.
Question. Where is the density of air higher; at compressions or at rarefactions?
Ans : At the compression the density of air is higher.
Question. Name the quantity that represents the length of one complete wave.
Ans : Wavelength represents the length of one complete wave.
Question. What is relation between time period and frequency?
Ans : Frequency Time period = 1
Question. Name two animals that communicate using infrasound?
Ans : Rhinoceroses and whales communicate using infrasound.
Question. Name the waves used by bats while flying in the dark.
Ans : Bats use ultrasonic waves while flying in the dark.
Question. What is the distance between two consecutive crests in a wave called?
Ans : Wavelength is the distance between two consecutive crests in a wave.
Question. Is the amplitude of a wave the same, as the amplitude of the vibrating body producing the wave?
Ans : Yes, the amplitude of a wave is same, as the amplitude of the vibrating body producing the wave.
Question. What is the range of frequencies associated with :
(a) Infrasound
(b) Ultrasound
Ans : (a) Infrasound : Sound waves between the frequencies 1 to 20 Hz.
(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.
Question. What is a stethoscope? Name the principle on which a stethoscope works.
Ans : Stethoscope is a medical instrument used for listening sounds produced within the body, chiefly in the heart or lungs. Stethoscope works on the principle of multiple reflection of sound.
Question. How moths of certain families are able to escape captures from bats? What is the range of frequencies associated with :
(a) infrasound?
(b) ultrasound?
Ans : They have very sensitive hearing equipment, that can hear the squeaks (ultrasound) of bat and know when a bat is flying nearby.
(a) Less than 20 Hz.
(b) More than 20,000 Hz.
Question. A person fires a gun standing at a distance of 55 m from a wall. If the speed of sound is 330 ms–1, find the time for an echo to be heard.
Ans : Given d = 55 m, v = 330 ms–1, t = ?
2d = v × t
or t = 2d/v== 2×55/330= 0.3
Question. What are wavelength, frequency, time period and amplitude of a sound wave?
Ans : Wavelength : It is the linear distance between two consecutive compressions or two consecutive rarefactions.
Frequency : The number of compressions or rarefactions taken together passing through a point in one second is called frequency.
Time period : It is the time taken by two consecutive compressions or rarefactions to cross a point.
Amplitude : It is the magnitude of maximum displacement of a vibrating particle about its mean position.
Question. The pulse rate of a man is 80 beats in one minute. Calculate its frequency.
Ans : No. of beats per minute = 80
No. of beats per second = 60 .
= 80 = 1 3
So, frequency = 1.3 Hz
Question. What is echo? Explain the conditions that have to be satisfied to hear an echo?
Ans : Reflection of sound wave from a large obstacle is called an echo. The most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound is off and the obstacle is at least at a distance of 17 m.
Question. Explain, why can echoes not be heard in a small room?
Ans : For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which sound is reflected. In small rooms this is not the case, hence, echoes are not heard.
Question. Which wave property determines :
(i) loudness (ii) pitch
Ans : (i) The amplitude of the wave determines the loudness. More the amplitude of a wave, more is the loudness produced.
(ii) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.
Question. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production
remains the same. Do you hear echo sound on a hotter day?
Ans : If the temperature rises, the speed of sound will increase. This in turn will increase the minimum distance required for hearing an echo. No echo is heard because the distance between the source of sound and reflecting body does not increase.
Question. Distinguish between transverse and longitudinal waves (three points).
Ans : Transverse waves :
(i) Particles the medium vibrate at right angles.
(ii) Alternate crests and troughs formed.
(iii) e.g., water waves.
Longitudinal waves :
(i) Particles vibrate parallel to the direction of waves.
(ii) Alternate compressions, rarefaction formed.
(iii) e.g., sound waves.
Question. Does sound follow the same laws of reflection as light does? Explain.
Ans : Yes, sound and light follow the same laws of reflection that are given below :
(a) Angle of incidence at the point of incidence =Angle of reflection. ^+i = +rh
(b) At the point of incidence, the incident sound wave, the normal and the reflected sound wave lie in the same plane.
Question. What is a trough?
Ans : A trough is a depression in a wave, i.e., maximum displacement in the negative direction (below the mean position).
Question. What do you understand by the term infrasonic vibrations?
Ans : The sounds of frequency lower than 20 Hz are called the infrasonics or subsonics.
Question. Which of the following sound waves we can hear : 10 Hz, 500 Hz, 1500 Hz, 12000 Hz, 25000 Hz?
Ans : 500 Hz, 1500 Hz, 12000 Hz.
Question. What are longitudinal waves? Give two examples.
Ans : A wave in which the particles of the medium vibrate back and forth along the same direction, in which the wave is moving, is called a longitudinal wave.
Examples :
(a) The sound waves in air.
(b) The waves produced in air when a sitar wire is plucked.
Question. State three characteristics of a musical sound. Onwhat factors do they depend?
Ans :Characteristics of musical sound are :
(i) Loudness–Amplitude affects loudness–more amplitude, louder the sound and lesser the amplitude, softer is the sound.
(ii) Pitch-Frequency affects pitch-more frequency more pitch, less frequency less pitch.
(iii) Quality.
Question. How does the sound produced by a musical instrument, reach your ears? Astronauts need radio transmitter to talk to each other on Moon. Why?
Ans :The sound produced by the musical instrument makes the molecules of air vibrate. These vibrations are carried forward by the other molecules till they reach our ear. These then vibrate our eardrum to producesound. Since, sound requires a medium to propagate,
therefore, sound cannot travel between astronauts on the Moon, hence, they use radio transmitters.
Question. State any two characteristics of a wave motion.
Ans : The characteristics of wave motion are :
(i) It is a periodic disturbance.
(ii) Energy transfer takes place at a constant speed.
Question. Will the sound be audible if the string is set into vibration on the surface of the Moon? Give reason for your answer.
Ans : No, we will not hear any audible sound on the surface of the Moon. This is because sound requires a medium to propagate, since there is no atmosphere on the surface of Moon, therefore, the sound will not be heard.
Question. What change, if any, would you expect in the characteristics of musical sound when we increase :
(i) its frequency, and
(ii) its amplitude?
Ans :
(i) Pitch of sound will increases,
(ii) Loudness of sound will increases.
Question. The stem of a tuning fork is pressed against a table top. Answer the following questions :
(i) Would the above action produce any audible sound?
(ii) Does the above action cause the table to set into vibrations?
(iii) If the answer above is yes, what type of vibrations are they?
(iv) Under what conditions does the above action lead to resonance?
Ans :
(i) Yes, there is an audible sound produced.
(ii) Yes, the table top is set into ‘forced vibrations’ by this.
(iii) The vibrations are forced vibrations.
(iv) Pressing the stem of a vibrating tuning fork against a table top, would lead to resonance if the frequency of the tuning fork equals the natural frequency of oscillations of the table top.
Question. If the amplitude of a wave is doubled, what will be the effect on its loudness?
Ans : Loudness depends upon the square of the amplitude of the wave, therefore, when the amplitude of wave is doubled, the loudness becomes four times.
Question. How do the frequency and amplitudes affect a musical sound?
Ans : The ‘frequency’ of a musical sound affects its ‘pitch’. The more the frequency of a (musical) sound, the ‘sharper’ and ‘shriller’ the sound becomes.
The ‘amplitude’ of a musical sound affects its loudness, or intensity. The more the amplitude of the sound, the louder (or more intense) the sound is.
Question. Give one example each of natural vibration, forced vibration and resonance.
Ans : (i) Natural vibration : The vibrations of a simple pendulum about its mean position.
(ii) Forced vibration : A sonometer wire, under tension, vibrating under the influence of a vibrating tuning fork.
(iii) Resonance : A correctly adjusted length of a sonometer wire under proper tension, vibrating under the influence of a vibrating tuning fork.
Question. Mention one practical use of echoes.
Ans : Echoes are used in radars to estimate the distance of flying objects.
Question. How does a stretched string on being set into vibration, produce the audible sound?
Ans : On being set into vibrations, the stretched string, forces the surrounding air to vibrate. This vibrating air, in turn, affects our eardrum and produces an audible sound.
Question. Write conditions for the production of an echo.
Ans : Conditions for the production of an echo are :
(i) Time gap between the original sound and the reflected sound.
The echo will be heard if the original sound reflected by an obstacle reaches our ears after 0.1 s.
(ii) Distance between the source of sound and obstacle.
Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
(iii) Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle must be rigid such as a building, hill or a cliff.
(iv) Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.
Question. A longitudinal wave is produced on a toy slinky. The wave travels at a speed of 30 cm/s and the frequency of the wave is 20 Hz. What is the minimum separation between the consecutive compressions of the slinky?
Ans : Wave speed, v = 30 cm/s
Frequency of the wave, v = 20 Hz = 20 s–1
The minimum separation between the consecutive
compressions is equal to the wavelength. Therefore,
Wavelength=30 cm s-1/20s-1= 1.5 cm
Question. A bat can hear sound at frequencies up to 120 kHz. Determine the wavelength of sound in the air at this frequency. Take the speed of sound in the air as 344 m/s.
Ans : Frequency,n = 120 kHz = 120 × 103 Hz
= 120 × 103 s–1
Velocity of sound in the air, v = 344 m/s
Wavelength of the sound wave = λ
We know,
Wavelength,λ= wave velocity/frequency
=344ms-1/120x103s-1
= 2.87 × 10–3 m = 0.29 cm
Question. Give uses of multiple reflection of sound.
Ans : There are several uses of multiple reflection of sound :
(i) Megaphone is a device used to address public meetings. It is horn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.
(ii) The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.
(iii) Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.
(iv) Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e., source of sound) placed at the focus of the sound board.
(v) Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).
(vi) Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.
Question. Give application of ultrasound (ultrasonic waves).
Ans : Ultrasonic waves have number of uses :
(1) Ultrasonic vibrations are used for homogenising milk. These vibrations break down the larger particles of the fat present in milk to smaller particles.
(2) Ultrasonic vibrations are used in dish washing machines. The vibrating detergent particles rub against the dirty utensils and thus, clean them.
(3) Ultrasonic vibrations produce a sort of depression in rats and cockroaches.
(4) Ultrasonic vibrations are used to study the growth of foetus in mother’s womb.
(5) Ultrasonic vibrations are used in relieving pain in joints and muscles.
(6) Ultrasonic vibrations are used in detecting flaws in articles made from metals. They are also used in finding the thickness of various parts of a metallic component.
Question. A tuning fork produces 1024 waves in 4 seconds.Calculate the frequency to the tuning fork.
Ans : As the tuning fork produces 1024 waves in 4 seconds, hence
Frequency of tuning fork,
n = Number of vibration per second
= 1024/4 = 256 Hz
Question. A human heart, on an average, is found to beat 75 times a minute. Calculate its frequency.
Ans : No. of beats of human heart = 75 min–1
= 75/1 min
= 75 /60 = 1.25 s–1
So, average frequency of human heart beating
= 1.25 s–1.
Question. A boat at anchor is rocked by waves whose consecutive crests are 100 m apart. The wave velocity of the moving crests is 20 m/s. What is the frequency of rocking of the boat?
Ans : Distance between two consecutive crests = 100 m
Wave velocity v = 20 m/s
The distance between two consecutive crests is equal to the wavelength of the wave. So,
Frequency = Wave velocity/Wave length
=20 ms-1/100 m= 0.2 s–1
So, the frequency of rocking of the boat is 0.2 s–1.
