9th Class Physics Comprehensive Questions Full Book
9th Class Physics Comprehensive Questions Full Book
Chapter 1
Q 1.1 What is meant by base and derived quantities ? Give the names and symbols of SI base units.
The quantities which are independent of the other physical quantities are called base quantities. We cannot define any base quantity in terms of any other quantity.
Derived quantities are the quantities which can be defined with reference to base quantities.
There are seven base physical quantities and hence seven base units. Symbol of meter is m, kilogram is kg, second is s, kelvin is K, ampere is A, candela is cd and mole is mol.
| Name of Unit | Symbol of Unit |
|---|---|
| Meter | m |
| Kilogram | kg |
| Second | s |
| Kelvin | K |
| Ampere | A |
| Candela | cd |
| Mole | mol |
Q 1.2 Give three examples of derived units in SI. How are they derived from base units ? Describe briefly.
Some of the derived physical quantities are area, volume and speed and there units are square meter, cubic meter and meter per second respectively.
When we multiply or divide two or more than two base units, we get derived units. Remember that by adding or subtracting base units, we cannot get a derived unit.
$$Area\;=\;Length\;\times\;breath\\=\;meter\;\times\;meter\\=\;Square\;meter\;=\;m^2$$
$$Volume=Length\times breath\times height\\=\;meter\;\times\;meter\;\times\;meter\\=\;Cubic\;meter\;=\;m^3$$
$$Speed\;=\;\frac{dis\tan ce}{time}\\=\frac{meter}{second}=\;m\;s^{-1}$$
Chapter 2
2.1:- How a vector can be represented graphically? Explain
We represent a vector by an arrow head on it or below it or with a bold face letter.
$$\boldsymbol A\boldsymbol\;or\;\overrightarrow A\;or\;\underrightarrow A$$
We represent the magnitude of vector by
$$A\;or\;\left|\boldsymbol A\right|\;or\;\left|\overrightarrow A\right|\;or\;\left|\underrightarrow A\right|$$
We represent a vector graphically by drawing a line segment with an arrow head at its one end. The length of line segment represents the magnitude of vector quantity according to a suitable scale and the direction of arrow represents the direction of vector. First we draw a cartesian coordinate system. A vector is drawn starting from origin towards the given direction. The direction is usually given by an angle with the x-axis. The angle with the x-axis is measured from the right side of x-axis in anti-clockwise direction.
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2.2:- Differentiate between rest and motion and speed and velocity.
Rest and Motion:-
Body is at rest, if a it does not change its position with respect to its surroundings.
Body is in motion, if it is continuously changing its position with respect to its surroundings.
The state of rest or motion of a body is always relative. Two passengers in a moving train are in rest with respect to each other but they are in a state of motion with respect to another observer standing at the platform.
Speed and Velocity:-
Distance covered in unit time in known as speed. It is a scalar quantity.
Speed = Distance/Time
v= s/t
Its SI unit is meter per second.
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2.4:- Explain the difference between distance and displacement.
The net displacement of a body in unit time is called velocity. It is a vector quantity.
If a body moves from a point A to a point B along a curved path, the displacement d is the straight line AB.
The direction of velocity is the same as the direction of displacement. It SI unit is also meter per second
Average velocity = Displacement/Time
$${\boldsymbol v}_{av}\;=\;\frac{\boldsymbol d}t\\Meter\;per\;second\;is\\denoted\;by\;ms^{-1}$$
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2.6:- Prove that the area under speed-time graph is equal to the distance covered by an object.
An object is moving with constant speed v. For a time interval t, the distance s covered by the object will vt.
The area under the graph for time interval t is the area of rectangles of sides t and v. This area is equal to v multiplied by t. So, the area under the speed-time graph up to time axis is numerically equal to the distance covered by the object in time t.
If speed increases uniformly from 0 to v in time interval t, area under the speed time graph is a triangle. Distance covered is equal to area under the graph.
Distance = 1/2 (height) (base) = 1/2 vt
if the speed increases uniformly from 0 to v in time interval t, distance covered is 1/2 vt, which is equal to area under the speed-time graph.
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Chapter 3
Q1:- Explain the concept of force by practical examples.
Force is an agency which changes or tries to change the state of an object. Force transfers energy to the object.
Pushing a door, kicking a ball, pulling a door and squeezing a ball are examples of force. Friction, gravity, compression and tension are also forces.
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Q2:- Describe Newton’s laws of motion.
Newton’s first law of motion:-
A object does not change its state of rest or motion in a straight line unless an external force is applied on it.
Newton’s first law is also called law of inertia. Inertia is that property of matter due to which it resists to change its state.
Examples of Newton’s first law of motion:-
A book on a table at rest will be at rest unless an external force is applied on it.
A car moving on a straight road will continue it unless an external force is applied on it
Newton’s second law of motion:-
When we apply a force on an object, it produces acceleration in it and this acceleration is directly proportional to the applied force and inversely proportional to the mass of that object.
$$a\;\propto F\\a\;\propto\frac1m\\After\;combining,\;we\;get\\a\propto\frac Fm\\a\;=\;k\;\frac Fm\\If\;\\a=\;1ms^{-2}\\F\;=\;1\;N\\m\;=\;1\;kg\\then\\K\;=\;1\\So\\a=\frac Fm\\of\\F\;=\;ma$$
Which is mathematical form of Newton’s second law of motion
Examples of Newton’s second law of motion:-
If we apply the same force on two balls, small ball will accelerate faster than a heavy ball.
Hitting a ball harder makes it go faster.
Newton’s Third law of motion:-
For every action, there is always an equal and opposite reaction.
It can also be expressed as of one body exerts a force on the second body. the second body also exerts an equal and opposite force on the first body.
Examples of Newton’s third law of motion:-
Lets the check the force on a book which is lying on the table. The force acting downward on the book is the weight. The book exerts a downward force on the table equal to its weight. The table also exerts a reaction force on the book.
When a bullet is fired from a gun, the bullet moves in the forward direction as an action and the gun recoils back as a reaction.
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Q3:- Define momentum and express Newton’s 2nd law of motion in terms of change in momentum.
The momentum of a moving body is the product of its mass and velocity.
P = m v
According to Newton’s Second law of motion.
$$F\;=\;m\;a\\F\;=\;m\;(\frac{\triangle v}{\triangle t})\\F\;=\;\frac{m\;(\;v_f\;-\;v_i)}{\triangle t}\\F\;=\frac{\;m\;v_f\;-\;m\;v_i}{\triangle t}\\F\;=\;\frac{p_f\;-\;p_i}{\triangle t}\\F\;=\;\frac{\triangle p}{\triangle t}$$
Equation shows that rate of change of momentum of a body is equal to the applied force.
Momentum is a vector quantity and its unit is Ns.
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Q4:- State and explain the principle of conservation of momentum.
Law of conservation of momentum states that for an isolated system momentum remains conserved.
Its mean that if no external force acts on an isolated system, the final momentum of the system is equal to the initial momentum of the system.
Let consider a system of two hard interacting balls.
$$Mass\;of\;1st\;ball\;=\;m_1\\Mass\;of\;2nd\;ball\;=\;m_2\\Velcoity\;of\;1st\;ball\;before\;collision=\;v_1\\Velcoity\;of\;2nd\;ball\;before\;collision\;=\;v_2\\Velcoity\;of\;1st\;ball\;after\;collision=\;v_1^\boldsymbol’\\Velcoity\;of\;2nd\;ball\;after\;collision=\;v_2^\boldsymbol’\\$$
When there is a collision of the two balls, there is a transfer of momentum from one ball to the other.
According to law of conservation of momentum
Momentum before the collision = momentum after the collision
$$\;m_1\;v_1+\;m_2\;v_2=\;\;m_1\;v_1^\boldsymbol’+\;m_2v_2^\boldsymbol’$$
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Q5:- Describe the motion of a block on a table taking into account the friction between the two surfaces. What is the static friction and kinetic friction ?
When block is placed on a table, two main forces act on it, weight in the downward direction and normal reaction in the upward direction. if we push it slightly, block will not move due to static friction between the block and the table. As we increase the applied force, the static friction also increases and attains its maximum limit.
When we apply a greater force then the maximum static friction, the block starts moving. When block moves, friction still exists. It is known as kinetic friction.
The force between two solid surfaces is called sliding friction. Sliding friction has two main categories.
Static Friction:-
The force that opposes the start of motion of a body at rest is called static friction. It has maximum value, after which the object will start moving.
$$static\;friction\;is\;denotes\;by\;F_s\;or\;f_s$$
Kinetic Friction:-
The friction which acts when object is moving is called kinetic friction. Its magnitude is generally less than maximum static friction.
$$kinetic\;friction\;is\;denotes\;by\;F_k\;or\;f_k$$
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Chapter 4
4.1 Explain the principle of moments with an example.
Principles of moments states that when a body is in balanced position, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point.
A child weighing 200 Newton sits 2 meter from the pivot on the left side. Another child sits on the right side at a distance of 4 meter from the pivot.
Anticlockwise moment = 200 x 2 = 400 Nm
So, the force on the right side is
F x 4 = 400
F = 100
Hence a child of 100 N sitting 4 m away balances the seesaw.
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4.2 Describe how could you determine the centre of gravity of an irregular shaped lamina experimentally.
Suspend the lamina from a point using a string and a weight to make a plumb line. Draw the line along the lamina’s surface. Repeat by suspending the lamina from different points. The intersection of these lines will give the centre of gravity of the lamina.
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Chapter 5
5.1 What is meant by kinetic energy? State its unit. Describe how it is determined.
The kinetic energy of a body is the energy that a body possesses by virtue if its motion. A object which is moving has the ability to do work due to its motion and this ability is called kinetic energy.
Kinetic energy formula:-
$$K.E=\frac12mv^2$$
Unit of kinetic energy is joule. We denote Joule by J. One joule of kinetic energy is the energy possessed by a body when a force of 1 newton moves it 1 meter in the direction of the force.
The work done on a body by a net force is equal to the change in its kinetic energy. If a force F moves a body through a distance d along the line of force, the work done w is
w= F.d
This work increases the body’s kinetic energy. K.E=w
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5.2 State the law of conservation of energy. Explain it with the help of an example of a body falling from certain height in terms of its potential energy and kinetic energy.
Law of conservation of energy states that energy can never be destroyed nor created. It may be transformed from one form to another, but the total amount of energy never changes.
Let a body of m be at rest at point “a” above the height h from the ground. At that point its kinetic energy will be zero and its potential energy will be mgh so its total energy will be mgh.
$$K.E_a=0\\P.E_a=mgh\\T.E_a=K.E_a\;+\;P.E_a\\=0+mgh=mgh$$
Now the body is allowed to drop to point “b” at a height x from the ground. This body will gain its kinetic energy and loss its potential energy. The loss of potential energy will appear as the gain in its kinetic energy.
$$K.E_b=mgx\\P.E_b=mg(h-x)\\T.E_b=K.E_b\;+\;P.E_b\\=mgx+mg(h-x)\\=mgx+mgh-mgx\\=mgh\\$$
Just before hitting the ground at point “c” all of its potential energy will be changed into kinetic energy.
$$K.E_c=mgh\\P.E_c=0\\T.E_c=K.E_c\;+\;P.E_c\\=mgh+0=mgh$$
Thus the total energy remains the same.
On hitting the ground, this energy is dissipated as heat energy and sound energy.
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5.4 Explain what is meant by efficiency of a machine. How is it calculated? Why there is a limit for the efficiency of a machine?
Efficiency of a machine is the ratio of useful output energy and the total input energy.
We can calculate the efficiency by dividing the useful output energy with the total input energy.
There is a limit for the efficiency of machine due to friction, air resistance, sound, deformation and heat loss.
Efficiency of a machine can never be 100 percent.
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Chapter 6
6.3 Describe Pascal’s law. State its applications with example.
When pressure is applied at one point in an enclosed fluid, it is transmitted equally to all parts of fluid without loss. Its examples are hydraulic press, hydraulic brakes, hydraulic jack and hydraulic lifts.
Hydraulic press
it is used to lift heavy objects. When a small force is applied on a small piston, a large force is produced on a large piston. It is used to extract oils from seeds.
Hydraulic Brakes
Hydraulic brakes are used in vehicles. When brake peddle is pressed, pressure is equally distributed through brake fluid to all the wheels.
Hydraulic lift is used to lift heavy loads like cars. Hydraulic jack is also used to lift heavy vehicles to change the tyre.
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6.4 On what factors the pressure of a liquid in a container depend? How is it determined?
The pressure of a liquid inside the container depends on depth of the liquid, density of the liquid and acceleration due to gravity.
Lets check how to determine the pressure
Consider an area A of the liquid at depth h. The force acting on the area is equal to the weight of the liquid column over surface A.
Hence, volume of the liquid is V=Ah
If rho is the density of liquid, then mass of the liquid column will be
$$m=\rho V=\rho Ah\\Therefore,\;force\;acting\;on\\area\;A\;will\;be\\F=mg=\rho Ah(g)\\Pressure\;p\;at\;the\;area\;A\\will\;be\\p=\frac FA=\frac{\rho Ah(g)}A$$
$$p=\rho gh\\Where\\p\;is\;pressure\;of\;the\;liquid\\\rho\;is\;density\;of\;the\;liquid\\g\;is\;gravitional\;acceleration\\and\;h\;is\;depth\;of\;the\;liquid$$
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Chapter 7
7.2 What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.
Temperature of a body is defined as the degree of hotness or coldness.
For the exact measurement of temperature, we require an instrument called thermometer.
Expansion in the volume of a liquid can be used for measurement. This is known as liquid-in-glass thermometer. Mercury is commonly used.
Mercury-in-glass thermometer is made of glass. It has bulb at one end filled with mercury. When the temperature rises, Mercury expands and moves up through the narrow capillary tube in the form of a mercury thread. The position of the end of the thread reads the temperature.
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7.3 Compare the three scales used for measuring temperature.
The three commonly used temperature scales are Celsius, Fahrenheit and Kelvin.
In Celsius or Centigrade scale, the numerical value assigned to lower and upper fixed points are 0 and 100 respectively. As the difference between these values is 100, so the space between these points is divided into 100 equal parts. each part is known as 1 degree centigrade.
In Fahrenheit scale, the numerical value assigned to lower and upper fixed points are 32 and 212 respectively. As the difference between these values is 180, so the space between these points is divided into 180 equal parts. each part is known as 1 degree Fahrenheit.
In Kelvin scale or Absolute temperature scale, the numerical value assigned to lower and upper fixed points are 273 and 373 respectively. As the difference between these values is 100, so the space between these points is divided into 100 equal parts. each part is known as 1 Kelvin.
Conversion of Celsius to Fahrenheit and Fahrenheit to Celsius:-
$$T_f=\frac95\times T_c+32\\T_c=\frac59(T_f-32)$$
Relationship between Kelvin and Celsius:-
$$T_k=T_c+273$$
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Chapter 8
8.1 How can you identify whether an object is a magnet or a magnetic material?
We can identify if an object is a magnet or a magnetic material with the help of repulsion test. We bring one end of the given object near a magnet. If the object repel the magnet, its a magnet. If it shows only attraction, its a magnet material.
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8.2 Describe the strength of a magnetic field in terms of magnetic lines of force. Explain it by drawing a few diagrams for the fields as examples.
Strength of magnetic field is described by the number of magnetic lines of forces passing through given area. These lines always go from North pole to South pole outside the magnet. The magnetic field is strong where the lines of force are close together and the magnetic field is weak where the lines of force are far apart.
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8.3 What is a circuit breaker? Describe its working with the help of a diagram.
Circuit breaker is a device which is used to an electric circuit by breaking the circuit whenever the current exceeds the safe limit.
When excess current flows, it produces a strong magnetic field in the electromagnet. This magnet field pulls the iron armature, causing the contacts to separate. Due to this, the circuit opens and current supply is cut off.
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8.4 A magnet attracts only a magnet. Explain the statement.
A magnet attracts only magnet is not a complete statement. Because a magnet can attract only magnet and magnetic material also. Magnwtic materials are not magnets themselves. Correct statement is that a magnet repels only a magnet.
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Chapter 9
9.1 What are the main branches of Physics? State briefly.
1) Mechanics :-
It is the study of motion.
2) Acoustics :-
it the study of audible sound energy.
3) Optics :-
It is the study of visible light.
4) Electromagnetism :-
It is the study of electromagnetic phenomena.
5) Quantum Mechanics :-
It is the study of particles at atomic and subatomic level.
6) Relativistic Mechanics :-
It explains how space and time are not absolute quantities but related to observer.
7) Nuclear Physics :-
It is the study of properties of nuclei of atoms.
8) Particle Physics :-
It is the study of subatomic and elementary particles.
9) Astronomy :-
It is the study of distribution of celestial bodies like planets and starts.
10) Cosmology :-
It tells us about the large structure and evolution of universe.
11) Solid State Physics :-
It is the study of properties of matter in solid form.
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9.3 What is scientific method? Describe its main steps with examples.
Scientific method is a scientific approach used to search for truth of an issue and problem solving regarding natural and physical word.
Lets check its main step with examples
1:- Observation
Careful watching and noting of phenomena or events.
For example plants near sunlight grow faster.
2:- Identification
After observation, we form a question
Does sunlight affect the growth of plant ?
3:- Hypothesis
A possible answer of the question.
Plants grow faster when they receive more sunlight.
4:- Experiment
We do experiments to test the hypothesis
We grow two plants. One in the sunlight and one in the shaded area and then compare their growth.
5:- Theory
A theory is a well tested explanation after repetition of experiments.
Theory explains how sunlight helps plants to grow.
6:- Prediction
A statement of what should happen if the theory is correct.
A plant receiving more sunlight will grow taller.
7:- Falsifiability
A scientific idea must be capable of being proven false by experiment.
If shaded plant grows taller, the idea is falsified.
8:- Scientific Law
A scientific law describes the natural relationship.
Plants grow faster with more sunlight.
Ohm’s law, law of inertia and law of conservation of energy are also a scientific laws.
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9.5 What is the scope of Physics in everyday life? Give some examples.
Physics is a fundamental science. Scope of Physics in everyday life is vast because it tells us how the universe works.
Medical applications:-
Physics is essential in medical technology for example X-rays and MRI.
Heat:-
Physics tells us about the phenomena of heat and temperature for example air conditioners and heaters.
Transport:-
Physics tells us about rest and motion for example a moving car and a rocket.
Electricity:-
Physics tells us how electricity flows for example phones and computers.
Light:-
Physics tells us how light travels for example camera and microscopes.
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