CBSE Class 9

On this page, you will find Sound Class 9 Notes Science Chapter 12 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 12 Sound will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 12 Notes Sound

Sound Class 9 Notes Understanding the Lesson

1. Sound
Sound is a form of energy which produces a sensation of hearing in our ears.

2. Production of sound
Sound is produced by vibration of objects.

• The sound of human voice is produced due to vibration in the vocal chord.

3. Propagation of sound
Sound propagates in the form of longitudinal waves and these waves require material medium to propagate. Hence sound waves are mechanical waves.

• A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion.
• The longitudinal wave of sound travels in the form of compression and rarefaction. Compression is the region of high pressure and rarefaction is the region of low pressure.
• The propagation of sound can be visualised as propagation of density variations or pressure variation in the medium.

4. Sound needs a medium to travel
Sound is a mechanical wave and need a material medium for its propagation.

• Longitudinal wave
  A wave in which the particles of the medium oscillate to and fro in the same direction in which the wave is moving is called longitudinal wave.
• Transversal wave
  A wave motion is said to transverse if the particles of the medium through which the wave propagates vibrate in the direction perpendicular to the direction of propagation of the wave.

5. Some important terms and Relations for longitudinal wave
Frequency: The frequency of wave is defined as the number of waves produced per second.
Or
The frequency of a sound wave is defined as the number of complete oscillations made by the particle of medium in one second.

• It is denoted by greek letter u (nu). Its SI unit is hertz (Hz).

(ii) Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is called wavelength.

• It is usually represented by X (Greek letter lambda). Its SI units is metre (m).

(iii) Time period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called time period of wave.
Or

• Time taken by particle of medium to complete one oscillation is known as time period.
• It is represented by the symbol T. Its SI unit is second (s).

8. Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called amplitude of wave. It is usually represented by the letter A. For sound its unit will be that of density or pressure.

9. Speed: The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefraction, travels per unit time.

10. Relations

• Relation between time period (T) and frequency (u)
  \(T=\frac{1}{v}\)
• Relation between speed of wave (υ), wavelength (λ) time period (T) and frequency (υ)
  υ = υλ

Sound propagates as density or pressure variations as shown in (a), (b) and (c) represents graphically the density and pressure variations.

11. Characteristics of a sound wave
We can describe a sound wave by

• Frequency
• Amplitude
• Speed

12. Frequency-pitch:

• How the brain interprets the frequency of an emitted sound is called its pitch. The faster the vibration of the source, the higher is the frequency and the higher is the pitch.
• The pitch of sound produced by an object of low frequency is low and the source described as flat sound.
• The pitch of sound produced by an object vibrating with high frequency is high and the sound is described as shrill sound.

13. Amplitude-loudness:

• The loudness or softness of a sound is determined by its amplitude. Greater the amplitude of vibration of source, greater is the loudness of sound.
• Loud sound can travel a larger distance as it is associated with high energy.
• A sound wave moves away from the source, its amplitude as well as its loudness decreases.

14. Quality-timber:

• The quality or timber of sound is that characteristics which enables us to distinguish one sound from an¬other having same loudness and pitch.
• A sound of single frequency is called a tone. The sound which is produced due to a mixture of several fre¬quencies is called a note and is pleasant to listen to.
• Quality of sound is represented by waveform.
• Noice is unpleasant to ear. Music is pleasant to hear and is of rich quantity.

15. Speed of sound in different media

• Speed of sound in a medium depends on inertia and elasticity of the medium.
• Speed of sound increases with increase in temperature.
• The speed of sound decreases when we go from solid to gaseous state.
  Speed of sound in solids > Speed of sound in liquids > Speed of sound in gases
• Speed of sound in air is 331 ms-1 at 0°C and 344 ms^-1 at 22°C.

16. Reflection of sound
Sound is reflected in same way as light. Incident and reflected ray make equal angles with the normal to the reflecting surface at the point of incidence and three are in the same plane.

17. Echo
An echo is the phenomenon of repetition of sound by reflection from an obstacle.

• The sensation of sound lasts in brain for (1/10) of a second. This property is called persistence of hearing. To hear a distinct echo the time interval between the original sound and the reflected one must be at least
  0. 1 second.
• For hearing a distinct echo, the minimum distance of the obstacle from the source of sound should be 17.2 m.

Take speed of sound = 344 m/s
(at temperature 20°C)
2d = v x t
v = 344 m/s
\(t=\frac{1}{10} \mathrm{s}\)
d=17.2m

18. Reverberation
A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in the persistence of sound is called reveberation.

Excessive reveberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound absorbent materials like compressed fibre board, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

Reflection of sound
d – distance of person from obstacle
v = velocity of sound
t = time after echo is heard
2d = v x t
t = 2 dlv and d = vt/2

19. Uses of multiple reflection of sound
(i) Megaphones, horns, musical instruments such as trumpets and shehnais are all designed to send sound in a particular direction. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of sound successively to guide most of sound from the source in the forward direction towards the audience.

2. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs

In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

3. Generally the ceiling of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall.
Sometimes curved sound board may be placed behind of the stage so that the sound after reflecting from the sound board, spreads evenly across the width of the hall.

20. Range of Hearing

• Audible range: The sound whose frequency lies between 20 Hz and 20,000 Hz which we are able to hear is called audible sound.
  Inaudible range
• Infrasonic sound: Sound of frequencies below 20 Hz is called infrasonic sound or infra sound.
• Ultrasonic sound: Frequency higher than 20 KHz is called ultrasonic sound or ultrasound.

21. Uses of Ultrasound in Communication
Sonar
The acronym sonar stands for sound navigation and ranging.
Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.

22. Components of sonar system
Sonar consists of a transmitter and a detector and is installed in a boat or a ship.

23. Working
The transmitter produces and transmits ultrasound waves. These waves travel through water and after striking the object on the sea bed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted.

24. Calculation of distance : The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound

Let the time intervel between transmission and reception of ultrasound signal be t and speed of sound through sea water be υ. The total distance, 2d, travelled by the ultrasound.

• Rhinoceroses communicate using infra sound of frequency as low as 5 Hz.
• Whales and elephants produce sound in infrasonic wave.
• Children under five and some animals, such as dogs can hear infrasonic sound.

Earthquake produces infrasonic waves.

• Ultrasound is produced by dolphines, bats and porpoises. R
• Moths of certain families can hear high frequency waves.

25. Application of Ultrasound
Industrial uses of ultrasound

(i) Cleaning instruments and electronic components
Ultrasound is generally used to clean parts located in hard to reach places, for example, spiral tube, odd shaped parts, electronic component, etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects are thus thoroughly cleaned.

(ii) Detecting flaw and cracks in metal blocks
Ultrasounds can be used to detect crack and flaws in metal blocks. Metallic components are generally used in the construction of big structures like buildings, bridges, machines and also scientific equipments. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of flaws or defect.

Medical uses of ultrasound
(i) Ultrasonography: The technique of obtaining of images of internal organs of the body by using ultrasonic waves is called ultrasonography. An ultrasound scanner is a medical instrument which is used by doctors to detect abnormalities such as stones in gall bladder and kidney or tumours in different organs. In this technique, the ultrasound scanner produces ultrasounds which travel through the tissues of the body, and if there are stones in the gall bladder or kidney or there is tumour in any internal organ, then the ultrasound waves get reflected from these regions due to the change in tissue density. These reflected ultrasound waves are converted into electrical signals and fed to the computer generating a three dimension images of the organ on the monitor of the computer.

(ii) Echocardiography: The technique of obtaining images of the heart by using reflection of ultrasonic waves from various parts of the heart is called echocardiography.

(iii) Breaking of kidney stones: Ultrasound can be used to break small stones formed in the kidney into fine grains. These grains later get flushed out of with urine.
2d = v x t

(iv) Use of ultrasound by bats for determining distance
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 pray is and what it is like.

26. Structure of the Human Ear
Introduction
The outer ear is called ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the eardrum or tympanic membrane. When 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 rarefraction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.

Class 9 Science Chapter 12 Notes Important Terms

Sound: Sound is a form of energy which produces a sensation of hearing in our ears.

Longitudinal wave: A wave in which the particles of the medium oscillate to and fro in the same direction in which the wave is moving is called longitudinal wave.

Transverse wave: A wave in which particles of the medium vibrate at right angles to the direction of the propagation of the the wave.

Echo: The repetition of sound caused by the reflection of sound waves is called an echo.

Reverberation: The persistence of sound in a big hall due to repeated reflection of sound from the walls, ceiling and floor of the hall is called reverberation.

Stethoscope: Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs.

Sonar: Sonar is a device that uses ultrasonic wave to measure the distance, direction and speed of underwater object.

ultrasonography: The technique of obtaining images of internal organs of the body by using echoes of ultrasound wave is called ultrasonography.

Echo cardiography: The technique of obtaining images of the heart by using reflection of ultrasonic waves from various parts of the heart is called echo cardiography.

On this page, you will find Work, Power And Energy Class 9 Notes Science Chapter 11 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 11 Work, Power And Energy will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 11 Notes Work, Power And Energy

Work, Power And Energy Class 9 Notes Understanding the Lesson

1. Work done by a constant force
Work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force.
Work done = force x displacement

• Work done has only magnitude and no direction i.e., work is a scalar quantity.
• SI unit of work is joule (J).
• 1 joule (one joule)

W = Fs
If F = 1 N and s = 1 m
W= 1 N x 1 Nm
W =  1 Nm
1j =1 Nm
1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

2. Conditions that need to be satisfied for work to be done

• Force should act on an object.
• The object must be displaced.

3. Zero work, Positive and Negative work
(i) Zero work: If the angle between force and displacement is 90°, then work done is said to be zero work.
Example: When a man carries a load on his hand and moves on a level road, work done by the man on the load is zero.

(ii) Positive work: Work done is said to be positive if force applied on an object and displacement are in the same direction.

Example: Work done by the force of gravity on a falling body is positive.

(iii) Negative work: Work done is said to be negative if the applied force on an object and displacement are in opposite direction.
W = -Fs
Here displacement is taken to be negative (-s).

Example: Work done by friction force is usually negative on a moving body.

4. Energy
Energy of a body is defined as the capacity or ability of a body to do work.
The SI unit of energy is joule (J) (unit of energy and work is same).

5. Forms of energy
There are various forms of energy in the nature, few of them are mechanical energy (potential energy + kinetic energy) heat energy, chemical energy and light energy.

6. Mechanical energy
Mechanical energy includes kinetic energy and potential energy.

7. Kinetic energy
The energy possessed by a body by the virtue of its motion is called kinetic energy.
Kinetic energy possessed by a body can be calculated by
\(E_{K}=\frac{1}{2} m v^{2}\)
m = mass of body
V = velocity of body

8. Derivation of kinetic energy (work energy theorem)
Let us consider an object lying on a frictionless surface having mass ‘m’

A force of constant magnitude F is acting on the body. Here initial velocity of the body is u and final velocity is v. As there is no dissipative forces, work done on the body will be stored in the form of change in kinetic energy.
W=Fs

If the object is starting from a stationary position u = 0, then

9. Potential energy
The energy possessed by a body due to its position or configuration is called potential energy.

10. Gravitational potential energy
Potential energy at any height (h) from a reference can be calculated by formula
E_p = mgh
where, m = mass of object
v = height from reference
The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.

11. Derivation of potential energy
When work is done on the body, the work is stored in the form of energy. Consider an object of mass, m. Let it be raised through a height, h from the ground. A force is required to do this. The minimum force required to raise the object is equal to the weight of the object, mg. The object gains energy equal to the work done on it. Let the work done on the object against gravity be W.

That is W = force x displacement
= mgh .
Since work done on the object is equal to mgh, an energy equal to mgh units is gained by the object. This is the potential energy (Ep) of the object.
E_p = mgh

12. Law of conservation of energy
Energy can neither be created nor be destroyed, it can only be transformed from one form to another. The total energy before and after the transformation always remains constant.

13. Transformation of energy in nature
The change of one form of energy into another form of energy is known as transformation of energy.
Example:

• Potential energy of water is converted into electricity in dams.
• Electricity is converted into heat energy in heaters.
• Chemical energy of fuel is converted into mechanical energy in engines.

14. Conservation of mechanical energy
Mechanical energy is the sum of kinetic energy and potential energy.
If there is no loss, then mechanical energy of a system is always constant.
Potential energy + kinetic energy = constant.
or
\(m g h+\frac{1}{2} m v^{2}=\text { constant }\)

15. Power (P)
Power is defined as the rate of doing work or rate of transfer of energy.
Power = work/time
P=W/T

• Unit of power is watt (W).

16. Watt

17. Commercial unit of energy
Kilowatt hour (kWh) or 1 unit
The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour.
1 kWh is the energy used in one hour (1 h) at the rate of 1000 J/s or (1 kW).
∴1 kWh =lkWxU = 1000 W x 3600 s = 3600000 J
1 kWh = 3.6 x 10⁶ J = 1 unit

18. Power can also be represented as,
P = Fv
F = force applied
v = velocity of object
\(P=\frac{W}{t}=\frac{F s}{t}=F v\)

Class 9 Science Chapter 11 Notes Important Terms

Work done: Work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force.

Energy: Energy of a body is defined as the capacity or ability of the body to do work.

Mechanical energy: Mechanical energy of a body is the sum of its kinetic energy and potential energy. Kinetic energy: The energy possessed by a body by the virtue of its motion.

Potential energy: The energy possessed by a body due to its position or configuration.

Law of conservation of energy: Energy can neither be created nor be destroyed, it can only be transformed from one form to another.

Conservation of mechanical energy: If there is no loss of energy, then mechanical energy of a system is always constant.

Power: Power is defined as the rate of doing work or rate of transfer of energy.

Commercial unit of energy: The energy used in households, industries and commercial establishment are usually expressed, in kilowatt hour. 1 kWh = 1 unit = 3.6 x 10⁶

On this page, you will find Gravitation Class 9 Notes Science Chapter 10 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 10 Gravitation will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 10 Notes Gravitation

Gravitation Class 9 Notes Understanding the Lesson

1. Newton’s law of gravitation
“Every particle in the universe attracts every other particle with a force, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between the two masses. The direction of force is along the line joining the two particles.”

Force of attraction between A and B

Where G is constant of proportionality called universal gravitational constant.

2. Universal gravitational constant
\(\mathrm{G}=\frac{\mathrm{F} r^{2}}{m_{1} m_{2}}\)
SI unit of G is Nm² kg^-2
Value of G = 6.67 x 10^-11 Nm² kg^-2.

3. Properties of gravitational force

• It is always attractive in nature.
• It obeys inverse square law  \(\mathrm{F} \alpha \frac{1}{r^{2}}\)
• Gravitational force is independent of the medium.
• It is a long range force.
• It is a weak force.
• Force of gravitation due to the Earth is called gravity.

4. Definition of G
Take = m2 = m = 1 kg r = 1 m
Hence, the force of attraction between two point masses separated by a unit distance is called universal gravitational constant.

5. Importance of the universal law of gravitation:
The universal law of gravitation successfully explained several phenomena

• The force that binds us to the Earth.
• The motion of the moon around the Earth.
• The motion of planets around the Sun; and
• The tides due to the moon and the Sun.

6. Free fall
Whenever objects fall towards the Earth under gravitational force alone, we say that the objects are in free fall.

7. Acceleration due to gravity (g):
The acceleration with which a body falls towards the Earth due to Earth’s gravitational pull is known as acceleration due to gravity. It is denoted by ‘g’.

8. Expression for acceleration due to gravity on the surface of the Earth:
The gravitational force between a body of mass ‘m’ and the Earth (of mass M) can be represented as
\(\mathbf{F}=\frac{\mathrm{G} \mathrm{Mm}}{r^{2}}\) …………….. (1)
Force of gravity is expressed as, F=Mg ……………(2)
From (1) and (2),\(g=\frac{\mathrm{GM}}{r^{2}}\)
The Earth is not a perfect sphere. As the radius of Earth increases from the poles to the equator, the value of g becomes greater at the poles than at the equator.

9. Calculate the value of g on the Earth:
\(g=\frac{G M}{r^{2}}\)
By putting the value of mass, radius of Earth and universal gravitational constant we can find out the value of acceleration due to gravity on the Earth.

10. Acceleration due to gravity at

• the surface of the Earth, g = 9.8 m/s²
• the centre of the Earth, g = 0.

11. Mass
Mass of a body is the quantity of matter contained in it.

• Mass of an object is constant and does not change from place to place.
• SI unit of mass is kilogram (kg).
• Mass of an object is a measure of its inertia.

12. Weight
The weight of an object is the force with which it is attracted towards the Earth.
w = mg

• SI unit of weight is newton (N).
• Weight is a vector quantity.
• Weight is directly proportional to mass of the body. So at a given place, weight of a body is a measure of its mass. w α m.
• Weight of a body changes from place to place because, acceleration due to gravity varies with position and location of a body.

13. Weight of an object on the moon
The acceleration due to gravity of moon is one sixth of Earth.
\(g_{\text {moon }}=\frac{1}{6} g_{\text {Earth }}\)
Due to this, the weight of an object on the moon is one sixth of the weight of the object on the Earth.
\(\frac{\text { Weight of the object on the moon }}{\text { Weight of the object on the Earth }}=\frac{1}{6}\)
Weight of the object on the moon = 1/6 x weight of the object on the Earth.

14. Gravitation (Flotation)
Thrust: The force acting on an object perpendicular to its surface is called thrust.

• SI unit of thrust is newton (N).

Pressure: The thrust on unit area is called pressure.

• SI unit of pressure is N/m² or Nm^-2.
• In honour of scientist Blaise pascal, the SI unit of pressure is called pascal (pa).
  1 pa = 1 N/m²
  Pressure = Thrust/Area

15. Consequences of pressure

(i) Nails and pins have painted ends so that these can be fixed with minimum force because the pressure on the painted ends would be large.

(ii) Wide wooden or metal or concrete sleepers are kept below railway lines to reduce pressure on the railway tracks and prevent them from sinking into the ground.

(iii) The foundation of a building or a dam has a large surface area so that the pressure exerted by it on the ground is less. This is done to prevent the sinking of the building or dam into the ground.

(iv) Skiers use flat skies to slide over snow as the long flat skies increase the area of contact, which reduces pressure exerted by the skier on the snow enabling the skier to slide over the snow without sinking.

(v) Broad handles are provided in bags and suitcases. Due to broad size of the handles, the area of contact increases which reduces the pressure exerted by the weight of the bag or suitcase.

(vi) A camel walks easily on the sandy surface than a man inspite of the fact that a camel is much heavier than a man. This is because their legs are padded and flat which provides greater surface area of contact with the sand and hence exerts less pressure on the sand. On the other hand, a man has very small surface area, so he exerts greater pressure and is likely to sink in sand.

16. Pressure in fluids
A substance which can flow is called a fluid. All liquid and gasses are fluids.

• A fluid contained in a vessel exerts pressure at all points of the vessel and in all directions.
• Pascal’s law: In an enclosed fluid, if pressure is changed in any part of the fluid, then this change in pressure is transmitted undiminished to all the other parts of the fluid.

17. Buoyancy
When a body is partially or wholly immersed in a fluid, an upward force acts on it which is called upthrust or buoyant force. The property of the fluids responsible for this force is called buoyancy.

18. Why do objects float or sink when placed on the surface of a fluid?

1. A body sinks if its weight is greater than the buoyant force acting on it.
Weight > buoyant force
Apparent weight = weight – buoyant force

2. A body floats if buoyant force balance the weight of the body.
A body having an average density greater than that of water (fluid), sinks into it while a body of average density smaller than that of water (fluid), floats on it.

19. Archimedes principle
When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

20. Density and relative density
Density: Density of a substance is defined as its mass per unit volume.
\(\text { Density }=\frac{\text { Mass }(\mathrm{M})}{\text { Volume }(v)} \text { or, } d=\frac{\mathrm{M}}{\mathrm{V}}\)
SI unit of density is kg/m³ or kg m^-3

21. Relative density: The relative density of a substance is the ratio of its density to that of water.
Relative density = Density of substance/Density of water.
It has no unit.

Class 9 Science Chapter 10 Notes Important Terms

Newton’s law of gravitation: Every particle in the universe attracts every other particle with a force, which is directly proportional to the product of their masses and inversely proportional to square of distance between the two masses.

Freefall: Whenever objects fall towards the Earth under gravitational force alone, we say that the objects are in free fall.

Acceleration due to gravity: The acceleration with which a body falls towards the Earth due to Earth’s gravitational pull is known as acceleration due to gravity.

Mass: Mass of a body is the quantity of matter contained in it.

Weight: The weight of an object is the force with which it is attracted towards the Earth.

Density: Density of a substance is defined as its mass per unit volume.

Relative density: The relative density of a substance is the ratio of its density to that of water.

Thrust: The force acting on an object perpendicular to the surface is called thrust.

Pressure: The thrust per unit area is called pressure.

Pascal’s law: In an enclosed fluid, if pressure is changed in any part of the fluid, then this change in pressure is transmitted undiminished to all the other parts of the fluid.

Buoyancy: When a body is partially or wholly immersed in a fluid, an upward force acts on it which is called upthrust or buoyant force.

Archimedes principle: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

On this page, you will find Force and Laws of Motion Class 9 Notes Science Chapter 9 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 9 Force and Laws of Motion will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 9 Notes Force and Laws of Motion

Force and Laws of Motion Class 9 Notes Understanding the Lesson

1. Force
It is entity which when applied on a body changes or tends to change a body’s

• state of rest
• state of uniform motion
• direction of motion
•  shape

2. Balanced forces
When a number of forces acting simultaneously on a body do not bring about any change in state of rest or of uniform motion along a straight line, then forces acting on a body are said to be balanced forces.

3. Unbalanced forces
When a number of forces acting simultaneously on a body bring about a change in its state of rest or of uniform motion along a straight line, then these forces acting on the body are said to be unbalanced forces.

4. Newton’s first law of motion
An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.

5. Inertia
Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.

6. The mass of an object is a measure of its inertia.
Examples:

• Passenger tends to fall backward, when a bus starts suddenly.
• Falling of fruits and leaves by a shaking tree.
• When a carpet is beaten with a stick, dust particles come out.
• When the card covering a glass tumbler is flicked with the finger coin placed over it falls in the tumbler.

7. Momentum (P)

• Momentum gives an idea about the quantity of motion contained in a body.
• The momentum (P) of an object is defined as the product of its mass (m) and velocity (v).
  P = mv
• Momentum is vector quantity and its unit is kg ms^-1.

8. Second law of motion
The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

9. Mathematical formulation of second law of motion
Suppose an object of mass, m is moving along a straight line with an initial velocity, v. It is uniformly accel-erated to velocity, v in time, t by application of constant force F throughout the time, t. The initial and final momentum of the object will be, P₁ = mu and P₂ = mv
respectively.


∴ F =ma
K is proportionality constant and the value So,
SI unit of force is newton.

10. First law of motion can be stated from the second law
F = ma
\(F=\frac{m(v-u)}{t}\)
Ft = mv – mu
when F = 0, v – u. This means that the object will continue moving with uniform velocity, u throughout the time, t. If u is zero then v will also be zero. That is, the object will remain at rest.

11. Example of second law of motion:
As we know that \(\text { F } \alpha \frac{1}{t}\)

• A cricket players lowers his hand while catching the ball to increase the time so that impact of force decreases.
• A karate player can break a pile of tiles with a single blow of his hand. This is because, as time decreases impact of force increases.
• Vehicles are fitted with shockers. The shockers increase the time of transmission of the force of the jerk to reach the floor of the vehicle. Hence less jerk is experienced by the passengers.

12. Third law of motion
According to the third law of motion, every action there is an equal and opposite reaction and they act on two different bodies.

Example of third law of motion:

• Recoiling of gun.
• When a man jumps out from a boat, the boat moves backward.

13. Conservation of momentum
In an isolated system (where there is no external force), the total momentum remains conserved.
m_Au_A + m_Bu_B = m_Av_A + m_Bv_B

14. Derivation
Suppose two objects of mass m_A and m_B are travelling in the same direction along a straight line at different velocity u_A and u_B respectively, and there are no external unbalanced forces acting on them. Let u_A > u_B and two balls collide each other. During collision which last for a time t, ball A exerts force on ball B and ball B exerts force F_BA on ball A. Suppose v_A and i>_B are the velocities after collision.

15. Illustration of conservation of momentum

• Recoil of gun.
• Rocket propulsion.
• Inflated balloon lying on the surface of a floor moves forward when pierced with a pin.

Class 9 Science Chapter 9 Notes Important Terms

Force: It is entity which when applied on a body changes or tends to change a body’s.

• state of rest
• state of uniform motion
• direction of motion
• shape

Balanced forces: When a number of forces acting simultaneously on a body do not bring about any change in state of rest or of uniform motion along a straight line, then forces acting on a body are said to be balanced forces.

Unbalanced forces: When a number of forces acting simultaneously on a body bring about change in its state of rest or of uniform motion along a straight line, then these forces acting on the body are said to be unbalanced forces.

Inertia: Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.

Momentum: Momentum of a body is product of its mass and velocity.

Conservation of momentum: In an isolated system the total momentum remains conserved.

On this page, you will find Motion Class 9 Notes Science Chapter 8 Pdf free download. CBSE NCERT Class 9 Science Notes Chapter 8 Motion will seemingly help them to revise the important concepts in less time.

CBSE Class 9 Science Chapter 8 Notes Motion

Motion Class 9 Notes Understanding the Lesson

1. Rest: A body is said to be at rest if its position does not change with respect to a fixed point taken as a reference point in its surrounding with the passage of time.

2. Motion: A body is said to be in motion if its position is changes with respect to a fixed point taken as a reference point in its surrounding with the passage of time.

3. Scalar quantity: A physical quantity which is described completely by its magnitude only, is called a scalar quantity.

4. Vector quantity: A physical quantity that has magnitude as well as direction is called a vector quantity. Distance: The total path length travelled by a body in a given interval of time is called distance. Displacement: The shortest distance measured from initial to the final position of an object is known as displacement.

5. Difference between distance and displacement

SI unit of distance and displacement is metre (m).

6. Uniform motion: A body moving in straight line has a uniform motion if it travels equal distance in equal intervals of time.

7. Non-uniform motion: A body has a non-uniform motion if it travels unequal distances in equal intervals of time.

8. Speed: The speed of a body is defined as distance travelled by it per unit time.

Speed is scalar quantity and SI unit of speed is m/s.

9. Average speed: It is defined as the total distance travelled by a body divided by the total time taken to cover this distance.

SI unit of speed and velocity is same.

Difference between speed and velocity

10. Acceleration: The rate of change of velocity of a body with respect to time is called its acceleration.

\(\text { Acceleration }=\frac{\text { Change in velocity }}{\text { time taken }}\)
\(a=\frac{v-u}{t}\)

Here, u – initial velocity
v = final velocity
t = time

11. Acceleration is a vector quantity and its SI unit is m/s².

12. If a body is travelling with uniform acceleration then its average velocity can be expressed as

\(v_{\text {avg }}=\frac{u+v}{2}\)
Here, u = initial velocity
v – final velocity

13. If the speed of a body is continuously increasing, the body is said to be continuously accelerating. If the speed of body is continuously decreasing, the body is said to be retarding.

14. Graphical Representation of Motion
Distance-time graph (Position time graph)

15. Slope of velocity-time graph gives speed and velocity

16. velocity-time graph

17. Slope of velocity-time graph gives distance and displacement.

18. Equation of motion by graphical method
Let us consider a body moving with acceleration a where u is initial velocity and v is final velocity, s is displacement of the object and t is time interval.

(i) v = u + at
We know that slope of v – t graph gives acceleration so slope
\(=a=\frac{v-u}{t-o}\)
\(\begin{array}{l}
a=\frac{v-u}{t} \\
v=u+\text { at }
\end{array}\)

(ii) \(s=u t+\frac{1}{2} a t^{2}\)
We know that area under v -1 graph gives displacement.
Area = s = area of triangle CDE + area of rectangle ABCE
\(s=u t+\frac{1}{2} \times t \times(v-u)\)
Putting the value of v – u.
\(s=u t+\frac{1}{2} \mathrm{at}^{2}\)

(iii) v² – u² = 2as
a where u is initial velocity and v is final velocity, s is displacement of the object and t is time interval.
\(s=\frac{1}{2} \times(v+u) \times t\)
\(\text { from } 1\left(t=\frac{v-u}{a}\right)\)
Putting the value of t.
v² – u² = 2as

19. Uniform circular motion
When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.
\(v=\frac{2 \pi r}{t}\)

If a body is moving in a circular path and completes one round (2πr distance) in time, speed is given by
Uniform circular motion is accelerated motion.

Class 9 Science Chapter 8 Notes Important Terms

State of rest: A body is said to be at rest if it does not change its position with respect to a fixed point taken as a reference point in its surroundings with passage of time.

State of motion: A body is said to be in motion if it changes its position with respect to a fixed point as a reference point in its surroundings with the passage of time.

Scalar quantity: A physical quantity which is described completely by its magnitude only, is called scalar quantity.

Vector quantity: A physical quantity which has magnitude as well as direction and obeys the vector addition is called vector quantity.

Distance: The total path length travelled by a body in a given interval of time is called distance.

Displacement: The shortest distance measured from initial to the final position of an object is known as displacement.

Uniform motion: A body moving in a straight line has a uniform motion, if it travels equal distance in equal intervals of time.

Non-uniform motion: A body has a non-uniform motion, if it travels unequal distances in equal intervals of time.

Speed: The speed of a body is defined as distance travelled by it per unit time.

Velocity: Velocity is defined as displacement per unit time.

Acceleration: The rate of change of velocity of a body with respect to time is called its acceleration.

Retardation: When acceleration of a body is opposite to its velocity, it is called retardation.

Uniform circular motion: When an object is moving in a circular path with a constant speed, the motion of the object is said to be uniform circular motion.