# 9th Physics Key Points

## 9th Class Physics

 Unit No. Unit / Chapter Name (Title) Page No. (Textbook) Unit 1 Physical Quantities and Measurement 1 Unit 2 Kinematics 27 Unit 3 Dynamics 55 Unit 4 Turning Effects of Forces 84 Unit 5 Gravitation 106 Unit 6 Work and Energy 117 Unit 7 Properties of Matter 143 Unit 8 Thermal Properties of Matter 167 Unit 9 Transfer of Heat 193

## 9th Physics – Full Syllabus Key Points

1st Chapter: Physical Quantities and Management

Key Points (Chapter No. 1)

1. Physics is a branch of science that deals with matter, energy, and their relationship.
2. Some main branches of physics are mechanics, heat, sound, light (optics), electricity and magnetism, nuclear physics, and quantities physics.
3. Physics plays an important role in our daily life. For example, electricity is widely used everywhere, domestic, appliance, office equipment’s, machines used in industry, means of transport and communication, etc. Work on the basic laws and principal of physics.
4. A measurement quantity is called physical quantity.
5. Base quantities are defined insistently. Seven quantities are selected base quantities. These are length, time, mass, electric current, temperature, the intensity of light, and the amount of a substance.
6. The quantities are expressed in terms of base quantiles are called derived quantities. For example, speed, area, density, force, pressure, energy, etc.
7. A worldwide system of measurements is known as the international system of units (SI). In SI, the unit of seven base quantities is matter, kilogram, second, ampere, kelvin, candela, and mole.
8. The words or letters are added before a unit and standards for the multiples or sub-multiples of that unit are known as prefixes. For example, kilo, mega, milli, micro, etc.
9. A way to express a given number as a number between 1 and 10 multiplied by 10 having an appropriate power is called scientific national or standard form.
10. An instrument used to measure small lengths such as internal or external diameter or length of a cylinder, etc is called Vernier Calipers.
11. A screw gauge is used to measure small lengths such as the diameter of a wire, the thickness of a metal sheet, etc.
12. Physical balance is a modified type of beam balance used to measure small masses by comparison with greater accuracy.
13. A stopwatch is used to measure the time interval of an event. Mechanical stopwatches have the least count up to 0.1 seconds. The digital stopwatch of least count 0.01s is common.
14. A measurement cylinder is a graduated glasses cylinder marked milliliters. It is used to measure the volume of a liquid and also to find the volume of an irregular-shaped solid object.
15. All the accurately known digits and the first doubtful digit in an expression are called significant figures. It reflects the precision of a measured value of a physical quantity.

2nd Chapter: Kinematics

Key Points (Chapter No. 2)

1. A body is said to be rest if it does not change its position with respect to its surroundings.
2. A body is said to be moved if it changes its position with respect to its surroundings.
3. Rest and motion are always relative. There is no such thing as absolute rest or absolute motion.
4. Motion can be divided into the following three types.
1. Translatory motion: In which a body moves without any rotation.
2. Rotatory motion: In which a body spins about its axis.
3. Vibratory motion: In which a body moves to and for about its mean position.
1. Physical quantities which are completely described by their magnitude only are known as scalars.
2. Physical quantities which are described by their magnitude a d direction are called vectors.
3. Position means the location of a certain place or object from a reference point.
4. The shortest distance between two points is called displacement.
5. The distance travelled in any direction by a body in unit time is called speed.
6. If the speed of a body does not change with time then its speed is uniform.
7. The average speed of a body is the ratio of the total distance covered to the total time taken.
8. We define velocity as rate of change of displacement or speed in a specific direction.
9. The average velocity of a body is defined as the ratio of its net displacement to the total time.
10. If a body covers equal displacements in equal intervals of time, however small the interval may be, then its velocity is said to be uniform.
11. The rate of change of velocity of a body is called acceleration.
12. A body has uniform acceleration if it has equal changes in its velocity in equal intervals of time, however small the interval may be.
13. A graph is a pictorial way of describing information as to how various quantities are related to each other.
14. The slope of the distance-time graph gives the speed of a body.
15. Distance–time graphs provide useful information about the motion of an object. The slope of the displacement – the time graph gives the velocity of the body.
16. The distance covered by a body is equal to the area under the speed-time graph.
17. A Speed-time graph is also useful for studying motion along a straight line.
18. The distance travelled by a body can also be found from the area under a velocity-time graph if the motion is along a straight line.
19. Equation of motion for uniformly accelerated motion is:

+ at

S     =  +  a

2as =  –

20. When a body is dropped freely it falls down with an acceleration towards the earth. This acceleration is called acceleration due to gravity and is donated by g. The numerical value of g is approximately 10m  near the surface of the earth.

3rd Chapter: Dynamics

Key Points (Chapter No. 3)

1. A force is a push or pull. It moves or tends to move, stops, or tends to stop the motion of a body.
2. The inertia of a body is its property due to which it resists any change in its state of rest or uniform motion in a straight line.
3. The momentum of a body is the quantity of motion possessed by the body. The momentum of a body is equal to the product of its mass and velocity.
4. The force that opposes the motion of a body is called friction.
5. Newton’s first law of motion states that a body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.
6. Newton’s second law of motion states that when a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, F = m a.
7. SI unit of force is newton (N). It is defined as the force which produces an acceleration of 1 in a body of mass 1 kg.
8. The mass of a body is the quantity of matter possessed by it. It is a scaler quantity. SI unit of mass is kilogramme (kg).
9. The weight of a body is the force of gravity acting on it. It is a vector quantity. SI unit of weight is newton (N).
10. Newton’s third law of motion states that to every action there is always an equal and opposite reaction.
11. The acceleration and tension in a system of two bodies attached to the end of a starting that passes over a frictionless pulley such that both move vertically are given by:

a =  g; T =  g

12. The acceleration and tension in a system of two bodies attached to the ends of a string that passes over frictionless pulley such that one moves vertically and the other moves on a smooth horizontal surface are given by:

a =  g; T =  g

13. Law of conservation of momentum states that the momentum of an isolated system of two or more than two interacting bodies remains constant.

14. A force between the sliding objects which opposes the relative motion between them is called friction.

15. Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than sliding friction.

16. The friction causes loss of energy in machines and much work has to be done in overcoming it. Moreover, friction leads to much wear and tear on the moving parts of the machine. The friction can be reduced by:

1. Smoothing sliding surface in contact.
2. Using ball bearing between the sliding surfaces.
3. Using ball bearing or roller bearing.
1. The motion of a body moving along a circular path is called circular motion.
2. The force which keeps the body moving in a circular path is called the centripetal force and given

By  =

1. According to Newton’s third law of motion, there exists a reaction to the centripetal force. A centripetal reaction that pulls the string outward is sometimes called the centrifugal force.

4th Chapter:  Turning Effect of Forces

Key Points (Chapter No. 4)

1. The parallel force has their lines of action to each other.
2. If the direction of parallel force is the same, they are called parallel force. If two parallel forces are in opposite directions to each other, then they are called, unlike parallel forces.
3. The sum of two or more forces is called the resultant force.
4. A graphical method used to find the resultant of two or more forces is called the head to tail rule.
5. Splitting up a force into two components perpendicular to each other is called resolution of that force. These components are

= F  ,  = F

1. A force can be determined from its perpendicular components are

F =  ,  =

1. Torque or moment of a force is the turning effect of the force. Torque of a force is equal to the product of force and moment arm of the force.
2. According to the principal of moments the sum of clockwise moments acting on a body sum of anticlockwise moments acting on it.
3. Center of mass of a body is such a point where a net force causes it to move without rotation.
4. The center of gravity of a body is a point where the whole weight of a body acts vertically downward.
5. A couple is formed by two parallel force of the same magnitude but acting in opposite directions along different lines of action.
6. A body is in equilibrium if net force acting on its is zero. A body in equilibrium either remains at rest or move with a uniform velocity.
7. A body is said to satisfy second condition for equilibrium if the resultant torque acting on it is zero.
8. A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position.
9. If a body does not return to its previous position when set free after slight tilt is said to be in unstable equilibrium.
10. A body that remains in its new position when disturbed from its previous position is said to be in a state of neutral equilibrium.

5th Chapter: Gravitation

Key Points (Chapter No. 5)

1. Newton’s law of universal gravitation states that everybody in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
2. The earth attracts a body with a force equal to its weight.
3. It is assumed that a gravitational field exits all around the earth due to the gravitational force of attraction of the earth.
4. In the gravitational field of the Earth, the gravitational force per unit mass is called the gravitational field strength of the earth. It is 10 near the surface of the earth.
5. Acceleration g = G
6. Mass of Earth =
7. G at an altitude h = G
8. An object that revolves around a planet is called a satellite.
9. The moon revolves around the earth so the moon is a natural satellite of the earth.
10. Scientists have sent many objects into space. Some of these objects revolve around the earth. These are called artificial satellites.
11. Orbital velocity =

6th Chapter:  Work and Energy

Key Points (Chapter No. 6)

1. Work is said to be done when a force acting on a body moves it in the direction of the force.
2. Work = FS
3. SI unit of work is joule (J).
4. When we say that a body has energy, we mean that it has the ability to do work. SI unit of energy is also joule, the same as work.
5. Energy exists in various forms such as mechanical energy, heat energy, light energy, sound energy electrical energy, chemical energy, and nuclear energy, etc. Energy from one form can be transformed into another.
6. The energy possessed by a body due to its motion is called kinetic energy.
7. The energy possessed body due to its position is called potential energy.
8. Energy cannot be created nor destroyed, but it can be converted from one form to another.
9. Processes in nature are the result of energy changes. The heat from the sun causes the water of oceans to evaporate to form clouds. As they cool down, they fall down as rain.
10. Einstein predicted the interconversion of matter and energy by the equation E = .
11. Fossil fuels are known as non-renewable resources because it took millions of years them to attain the present form.
12. Sunlight and water power the renewable resources of energy. They will not run out like coal, oil and gas.
13. Environmental problems such as polluting emission consisting of noise, air pollution and water pollution may arise by using different sources of energy such as fossil fuels, nuclear energy.
14. The ratio of the useful work by a device or machine to the total energy take up by it is called its efficiency.
15. Power is defined as the rate of doing work.
16. The power of a body is one wait which is doing work at the rate of one joule per second.

7th Chapter: Properties of Matter

Key Points (Chapter No. 7)

1. The kinetic energy molecular model explains the three states of matter assuming that
1. The matter is made up of particles called molecules.
2. The molecules remain in continuous motion.
1. AT a very high temperature, the collision between atoms and molecules tears off their electrons. Atoms become positive ions. This ionic state of matter is called the plasma-the fourth state of matter.
2. Density is the ratio of mass to volume of a substance. The density of water is 1000.
3. Pressure is the normal force acting per unit area. Its SI unit is or pascal (Pa).
4. The instruments that measure atmospheric pressure are called barometers.
5. The atmospheric pressure decreases as we go up. Thus, knowing the atmospheric pressure of a place, we can determine its altitude.
6. The changes in atmospheric pressure at a certain place indicate the expected changes in the weather condition of that place.
7. Liquid also exert pressure given by: P = p g h
8. Liquids transmit pressure equally in all directions. This is called pascals law.
9. When a body is immersed wholly or partially in a liquid, it loses its weight equal to the weight of the liquid displaced. This is known as Archimedes principle.
10. For an object to float, its weight must be equal to or less than the upthrust of the liquid acting on it.
11. The property of matter by virtue of which matter resists any force which ties to change its length, shape, or volume is called elasticity.
12. Stress is the deforming force acting per unit area.
13. The ratio of change of length to the original length is called tensile strain.
14. The ratio between stress and tensile strain is called Young’s modulus.

8th Chapter: Thermal Properties of Matter

Key Ponts (Chapter No. 8

1. The temperature of a body is the degree of hotness or coldness of the body.
2. Thermometers are made to measure the temperature of a body or place.
3. The lower fixed point is the mark that gives the position of mercury in the thermometer when it is placed in ice.
4. The upper fixed point is a mark that shows the position of mercury in the thermometer when it is placed in a stream from boiling water at standard pressure.
5. Inter-conversion between scales:
1. From Celsius To Kelvin Scale:

T (K) = 273 + C

1. From Kelvin To Celsius Scale:

C = T (K) – 273

1. From Celsius To Fahrenheit Scale:

F = 1.8C + 32

1. Heat is a form of energy and this energy is called heat as long as it is in the process of transfer from one body to another body. When a body is heated, the kinetic energy of its molecules increases, the average distance between the molecules increase.
2. The specific heat of a substance is defined as the amount of heat required to raise the temperature of a unit mass of that substance through one-degree centigrade (1 ) or one kelvin (1K).
3. The heat required by the unit mass of a substance at its melting point to change it from solid-state to a liquid state is called the latent of fusion.
4. The quantity of heat required by the unit mass of a liquid at a certain constant temperature to change its state completely from a liquid into a gas is called the latent heat of vaporization.
5. It has been observed that solids expand on heating and their expansion is nearly uniform over a wide range of temperature Mathematically,

L =  (1 +

1. The thermal coefficient of liner expansion of substance is defined as the fractional increase in its length per kelvin rise in temperature.
2. The volume of solid changes in temperature and is called volume or cubical expansion.

V =  (1 + B

1. The thermal coefficient of volume expansion B is defined as the fractional change in its volume per kelvin change in temperature.
2. There are two types of thermal volume expansion for liquid as well as for gases. Apparent volume expansion and real volume expansion.

9th Chapter: Transfer of Heat

Key Points (Chapter No. 9)

1. Heat flows from a body at a higher temperature to a body at a lower temperature.
2. There are three ways of heat transfer. These are conditions, convection radiation.
3. The mode of transfer of heat by vibrating atoms and free electrons in solids from hotter to a colder part of a body is called the conduction of heat.
4. The amount of heat that flows in unit time is called the rate of flow of heat.
5. The rate at which heat flows through solids depends on the cross-sectional area of the solid, length between hot and cold ends, the temperature difference between hot and cold end, and the nature of the martial.
6. The rate of flow of heat across the opposite faces of the meter cube maintained at the difference of 1 K is called the thermal conductivity of the material of the cube.
7. Good conductors are used for the quick transfer of heat. Thus cookers, cooking plates, boilers, radiators and condensers of retreaters, etc. are made of metals.
8. Water is a poor conductor of heat.
9. Materials that trap air are also bad conductors such as wool, felt, fur, feathers, polystyrenes, and fiberglass.
10. Transfer of heat by actual movement of molecules from a hot place to a cold place is known as convection.
11. Land and sea breezes are also examples of convection.
12. Gliders use the upward movement of hot air currents due to the convection of heat. Air currents help them to stay in the air for a long period.
13. Birds are able to fly for hours without flapping their wings due to the upward movement of air currents.
14. The term radiation means the continual emission of energy from the surface of a body in the form of electromagnetic waves.
15. Radiations are emitted by all bodies. The rate at which radiations are emitted depends on various factors such as colour and texture of the surface, temperature, and surface area.
16. A dull black surface is a good absorber of heat as its temperature rises rapidly
17. A polished surface is a poor absorber of heat its temperature rises very slowly.
18. Radiations from the sun pass easily through glass/polyethylene and warm up the materials inside a greenhouse. The radiations given out by them are of much longer wavelengths. Glass/polythene does not allow them to escape out and thus maintains the inside temperature of the greenhouse.
19. Earth’s atmosphere contains carbon dioxide and water vapors. It causes the greenhouse effect and thus retains the temperature of the earth.
20. The bottoms of cooking pots are made black to increase the absorption of heat from fire.
21. Whit surface reflect more heat than coloured or black surfaces. Similarly, polished surfaces are a good reflection than rough surfaces, and the reflection of heat radiations is greater than polished surfaces. Therefore, we wear white or light coloured clothes in summer.
22. We polish the interior of the cooking pots for reflecting black most of the heat radiation inside the hot pots.
23. A thermos flask consists of a double-walled glass vessel. It reduces the transfer of heat by conduction, convection, and radiation.