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      Class 9 Science

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      • Class 09
      • Class 9 Science
      CoursesClass 09ScienceClass 9 Science
      • 01.Matter in Our Surroundings
        9
        • Lecture1.1
          Matter and Its Properties 39 min
        • Lecture1.2
          States of Matter and Compressibility & Rigidity 21 min
        • Lecture1.3
          Moving through, Fluidity, Density, Diffusion, Pressure 26 min
        • Lecture1.4
          Change of State of Matter 56 min
        • Lecture1.5
          Evaporation and Boiling 39 min
        • Lecture1.6
          Cooling Effect of Evaporation Two More States of Matter 22 min
        • Lecture1.7
          Chapter Notes – Matter in Our Surroundings
        • Lecture1.8
          NCERT Solutions – Matter in Our Surroundings
        • Lecture1.9
          Revision Notes Matter in Our Surroundings
      • 02.Is Matter Around Us Pure
        9
        • Lecture2.1
          Elements and Compounds 20 min
        • Lecture2.2
          Mixtures, Homogeneous & Heterogeneous and Chemical & Physical Change 37 min
        • Lecture2.3
          Solution, Suspension, Colloid and Tyndal Effect 48 min
        • Lecture2.4
          Terms related to Solution and Solubility 42 min
        • Lecture2.5
          Separation of Mixtures 47 min
        • Lecture2.6
          Cleaning Drinking Water and Classification of Matter 29 min
        • Lecture2.7
          Chapter Notes – Is Matter Around Us Pure
        • Lecture2.8
          NCERT Solutions – Is Matter Around Us Pure
        • Lecture2.9
          Revision Notes Is Matter Around Us Pure
      • 03.Atoms and Molecules
        7
        • Lecture3.1
          Atom and its Properties, Atomic Mass, Molecule and Molecular Mass 40 min
        • Lecture3.2
          Ions, Valency and Writing the formulae of Compounds 47 min
        • Lecture3.3
          Mole Concept 42 min
        • Lecture3.4
          Calculation of no. of Moles, Percentage of an Element in a Compound and Laws of Chemical Combination 30 min
        • Lecture3.5
          Chapter Notes – Atoms and Molecules
        • Lecture3.6
          NCERT Solutions – Atoms and Molecules
        • Lecture3.7
          Revision Notes Atoms and Molecules
      • 04.Structure of The Atom
        7
        • Lecture4.1
          Charged Particles in Matter and Discovery of Sub – atomic Particles 31 min
        • Lecture4.2
          Atomic Model by J.J Thomson’s Model and Rutherford 57 min
        • Lecture4.3
          Ion, Mass Number, Isotopes, Isobar and Fractional Mass number of Elements 01 hour
        • Lecture4.4
          Electron Distribution in Orbits and Valency 01 hour
        • Lecture4.5
          Chapter Notes – Structure of The Atom
        • Lecture4.6
          NCERT Solutions – Structure of The Atom
        • Lecture4.7
          Revision Notes Structure of The Atom
      • 05.Cell - Fundamental Unit of Life
        7
        • Lecture5.1
          Introduction to Cell and Its Structure – Cell Wall 31 min
        • Lecture5.2
          Cell Membrane, Cytoplasm, Nucleus 32 min
        • Lecture5.3
          Ribosomes, Endoplasmic Reticulum, Golgi bodies, Lisosomes, Mitocondria 38 min
        • Lecture5.4
          Plastids, Vacuole; Types of Cell and Transport through Plasma Membrane 33 min
        • Lecture5.5
          Chapter Notes – Cell – Fundamental Unit of Life
        • Lecture5.6
          NCERT Solutions – Cell – Fundamental Unit of Life
        • Lecture5.7
          Revision Notes Cell – Fundamental Unit of Life
      • 06.Tissues
        8
        • Lecture6.1
          Division of Labour, Plant Tissues – Merismatic Tissue 20 min
        • Lecture6.2
          Plant Tissue-Permanent Tissue: Simple Permanent Tissue 34 min
        • Lecture6.3
          Plant Tissue-Permanent Tissue: Complex and Protective Tissue 31 min
        • Lecture6.4
          Animal Tissue- Muscular Tissue and Nervous Tissue 27 min
        • Lecture6.5
          Animal Tissue- Connective Tissue and Epithelial Tissue 42 min
        • Lecture6.6
          Chapter Notes – Tissues
        • Lecture6.7
          NCERT Solutions – Tissues
        • Lecture6.8
          Revision Notes Tissues
      • 07.Diversity in Living Organisms
        8
        • Lecture7.1
          Introduction to Biodiversity and Classification, Nomenclature 35 min
        • Lecture7.2
          The Five Kingdoms 29 min
        • Lecture7.3
          Plantae Kingdom 42 min
        • Lecture7.4
          Animalia Kingdom 01 hour
        • Lecture7.5
          Classification of Chordates and Revision of Animalia Kingdom 33 min
        • Lecture7.6
          Chapter Notes – Diversity in Living Organisms
        • Lecture7.7
          NCERT Solutions – Diversity in Living Organisms
        • Lecture7.8
          Revision Notes Diversity in Living Organisms
      • 08.Motion
        11
        • Lecture8.1
          Straight Line Motion, Position, Vector and Scalar; 23 min
        • Lecture8.2
          Distance & Displacement 48 min
        • Lecture8.3
          Speed and Velocity 57 min
        • Lecture8.4
          Acceleration and Deacceleration, Uniform & Non-uniform Acceleration 47 min
        • Lecture8.5
          Equations of Uniformly Accelerated Motion 58 min
        • Lecture8.6
          Uniform Circular Motion 30 min
        • Lecture8.7
          Graphical representation of Motion 30 min
        • Lecture8.8
          Calculations Using Graph 28 min
        • Lecture8.9
          Chapter Notes – Motion
        • Lecture8.10
          NCERT Solutions – Motion
        • Lecture8.11
          Revision Notes Motion
      • 09.Force and Newtons Laws of Motion
        12
        • Lecture9.1
          Force and Net or Resultant Force 46 min
        • Lecture9.2
          Newton’s 1st Law 46 min
        • Lecture9.3
          Momentum 46 min
        • Lecture9.4
          Newton’s 2nd Law 27 min
        • Lecture9.5
          Newton’s 2nd Law in Terms of Acceleration; 42 min
        • Lecture9.6
          Newton’s 1st Law from 2nd law and Practical Examples of Newton’s 2nd Law 42 min
        • Lecture9.7
          Newton’s 3rd Law and its Practical Examples 35 min
        • Lecture9.8
          Conservation of Momentum 33 min
        • Lecture9.9
          Practical Applications of Conservation of Momentum 29 min
        • Lecture9.10
          Chapter Notes – Force and Newtons Laws of Motion
        • Lecture9.11
          NCERT Solutions – Force and Newtons Laws of Motion
        • Lecture9.12
          Revision Notes Force and Newtons Laws of Motion
      • 10.Gravitation
        9
        • Lecture10.1
          Universal Law of Gravitation and Free Fall 41 min
        • Lecture10.2
          Falling Bodies: case 1 46 min
        • Lecture10.3
          Falling bodies: case 2 and Air Resistance 42 min
        • Lecture10.4
          Value of g, Mass and Weight, Weight on Moon, Circular Motion and Gravitation 48 min
        • Lecture10.5
          Thrust and Pressure and Pressure in fluids 37 min
        • Lecture10.6
          Archimedes Principle; Density 37 min
        • Lecture10.7
          Chapter Notes – Gravitation
        • Lecture10.8
          NCERT Solutions – Gravitation
        • Lecture10.9
          Revision Notes Gravitation
      • 11.Work and Energy
        7
        • Lecture11.1
          Work and Workdone Against Gravity, Oblique Displacement 01 hour
        • Lecture11.2
          Energy and Different Forms of Energy, Derivation of Formulae of K.E. 58 min
        • Lecture11.3
          Transformation of Energy and Law of Conservation of Energy 01 hour
        • Lecture11.4
          Rate of doing Work and Commercial Unit of Energy 43 min
        • Lecture11.5
          Chapter Notes – Work and Energy
        • Lecture11.6
          NCERT Solutions – Work and Energy
        • Lecture11.7
          Revision Notes Work and Energy
      • 12.Sound
        8
        • Lecture12.1
          Production and Propagation of Sound and Medium for Sound 30 min
        • Lecture12.2
          Frequency and Velocity of Sound Wave, Characteristic of Sound 01 hour
        • Lecture12.3
          Sonic Boom, Light faster than Sound, Reflection of Sound, Echo and Reverberation 56 min
        • Lecture12.4
          Types of Sound Based on Frequency 42 min
        • Lecture12.5
          Human Ear and Intensity of Sound 25 min
        • Lecture12.6
          Chapter Notes – Sound
        • Lecture12.7
          NCERT Solutions – Sound
        • Lecture12.8
          Revision Notes Sound
      • 13.Why do We Fall Ill
        7
        • Lecture13.1
          Factors Determining Health, Disease, Look of Disease, Cause of Disease, Types of Diseases 24 min
        • Lecture13.2
          Infectious Diseases, Immune System, Treatment of Diseases 55 min
        • Lecture13.3
          Prevention from Disease, Vaccination – Immunisation 36 min
        • Lecture13.4
          AIDS, Different Modes for Different Diseases 18 min
        • Lecture13.5
          Chapter Notes – Why do We Fall Ill
        • Lecture13.6
          NCERT Solutions – Why do We Fall Ill
        • Lecture13.7
          Revision Notes Why do We Fall Ill
      • 14.Natural Resources
        7
        • Lecture14.1
          Natural Resource-Air and its Pollution 57 min
        • Lecture14.2
          Natural Resource-Water and Soil and its Pollution 49 min
        • Lecture14.3
          Biogeochemical Cycles 33 min
        • Lecture14.4
          Green House Effect, Global Warming and Ozone Layer Depletion 30 min
        • Lecture14.5
          Chapter Notes – Natural Resources
        • Lecture14.6
          NCERT Solutions – Natural Resources
        • Lecture14.7
          Revision Notes Natural Resources
      • 15.Improvements in Food Resources
        7
        • Lecture15.1
          Various Revolution Held in India; Types of Crops; Crop Season; Crop Variety Improvement 31 min
        • Lecture15.2
          Crop Production Improvement 45 min
        • Lecture15.3
          Crop Protection Improvement and Organic Farming 29 min
        • Lecture15.4
          Animal Husbandry 40 min
        • Lecture15.5
          Chapter Notes – Improvements in Food Resources
        • Lecture15.6
          NCERT Solutions – Improvements in Food Resources
        • Lecture15.7
          Revision Notes Improvements in Food Resources

        NCERT Solutions – Work and Energy

        Intext Questions

        Q.1     A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
                                             1                                                          

        Sol.     When a force F acts on an object to displace it through a distance S in its direction,
        then the work done W on the body by the force is given by:
        Work done = Force × Displacement
        W = F × S
        Where,
        F = 7 N
        S = 8 m
        Therefore, work done, W = 7 × 8
        = 56 Nm
        = 56 J


        Page 149

        Q.1    When do we say that work is done?
        Sol.     Work is done whenever the given conditions are satisfied:
        (i) A force acts on the body.
        (ii) There is a displacement of the body caused by the applied force along the direction of the applied force.


        Q.2     Write an expression for the work done when a force is acting on an object in the direction of its displacement.
        Sol.     When a force F displaces a body through a distance S in the direction of the applied force, then the work done W on the body is given by the expression:
                   Work done = Force × Displacement
        W = F × s8080


        Q.3     Define 1 J of work.
        Sol.         1 J is the amount of work done by a force of 1 N on an object that displaces it through a distance of 1 m in the direction of the applied force. 80


        Q.4     A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
        Sol.     Work done by the bullocks is given by the expression:
                   Work done = Force × Displacement
                   W = F × d
        Where,
        Applied force, F = 140 N
        Displacement, d = 15 m
        W = 140 × 15 = 2100 J
        Hence, 2100 J of work is done in ploughing the length of the field.


        Page 152

        Q.1     What is the kinetic energy of an object?
        Sol.      Kinetic energy is the energy possessed by a body by the virtue of its motion. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.


        Q.2     Write an expression for the kinetic energy of an object.
        Sol.      If a body mass m is moving with a velocity v, then its kinetic energy Ek is given by the expression, 
                   Ek=12mv2. Its SI unit is Joule (J).


        Q.3     The kinetic energy of an object of mass, m moving with a velocity of 5 m s−1 is 25 J.What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
        Sol.         Expression for kinetic energy Ek=12mv2
                    m = Mass of object 
                    v = Velocity of the object ms−1
                    Given that kinetic energy, Ek=25J

                    (i) If the velocity of an object is doubled, then v = 5 × 2 = 10 ms−1.
        Therefore, its kinetic energy becomes 4 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 4 = 100 J.
                    (ii) If velocity is increased three times, then its kinetic energy becomes 9 times its original value, because it is proportional to the square of the velocity. Hence, kinetic energy = 25 × 9 = 225 J.80


        Page 156

        Q.1     What is power?
        Sol.     Power is the rate of doing work or the rate of transfer of energy. If W is the amount of work done in time t, then power is given by the expression,
                   Power=WorkTime=EnergyTime
                   P=WT
                   It is expressed in watt (W).


        Q.2     Define 1 watt of power:
        Sol.          A body is said to have power of 1 watt if it does work at the rate of 1 joule in 1 s,i.e.,
                   1W=1J1s


        Q.3     A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
        Sol.     Power is given by the expression,
                   Power=WorkdoneTime
                   Work done = Energy consumed by the lamp = 1000 J 
                   Power=100010=100Js−1
                   = 100 W


        Q.4     Define average power.
        Sol.     A body can do different amount of work in different time intervals. Hence, it is better to deϐine average power. Average power is obtained by dividing the total amount of work done in the total time taken to do this work.
                   AveragePower=Totalworkdone/Totaltimetaken

         

        Exercise

        Q.1     Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.Suma is swimming in a pond. A donkey is carrying a load on its back. A wind mill is lifting water from a well. A green plant is carrying out photosynthesis.An engine is pulling a train. Food grains are getting dried in the sun. A sailboat is moving due to wind energy.
        Sol.     Work is done whenever the given two conditions are satisfied:
                   (i) A force acts on the body.
                   (ii) There is a displacement of the body by the application of force in or opposite to the direction of force.
                   (a) While swimming, Suma applies a force to push the water backwards. Therefore, Sumaswims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Seema while swimming.
                   (b) While carrying a load, the donkey has to apply a force in the upward direction.But, is placement of the load is in the forward direction. Since, displacement is perpendicular to force,the work done is zero.
                   (c) A wind mill works against the gravitational force to lift water. Hence, work is done by the wind mill in lifting water from the well.
                   (d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero.
                   (e) An engine applies force to pull the train. This allows the train to move in the direction of force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train.
                   (f ) Food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in the Sun.
                   (g) Wind energy applies a force on the sailboat to push it in the forward direction.Therefore, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.


        Q.2     An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
        Sol.      Work done by the force of gravity on an object depends only on vertical displacement.Vertical
        displacement is given by the difference in the initial and final positions/heights of the object, which is
        zero. Work done by gravity is given by the expression,
        W = mgh
        Where,
        h = Vertical displacement = 0
        W = mg × 0 = 0 J
        Therefore, the work done by gravity on the given object is zero joule.


        Q.3     A battery lights a bulb. Describe the energy changes involved in the process.
        Sol.     When a bulb is connected to a battery, then the chemical energy of the battery is transferred into electrical energy. When the bulb receives this electrical energy, then it converts it into light and heat
        energy. Hence, the transformation of energy in the given situation can be shown as:


        Q.4     Certain force acting on a 20 kg mass changes its velocity from 5 ms−1 to 2 ms−1. Calculate the work done by the force.
        Sol.       Kinetic energy is given by the expression,


        Q.5     A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
        Sol.     Work done by gravity depends only on the vertical displacement of the body. It does not depend upon the path of the body. Therefore, work done by gravity is given by the expression,
        W = mgh
        Where,
        Vertical displacement, h = 0
        ∴W = mg × 0 = 0
        Hence, the work done by gravity on the body is zero.


        Q.6     The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
        Sol.       No. The process does not violate the law of conservation of energy. This is because when the body falls from a height, then its potential energy changes into kinetic energy progressively. A decrease in the potential energy is equal to an increase in the kinetic energy of the body. During the process, total mechanical energy of the body remains conserved. Therefore, the law of conservation of energy is not violated.


        Q.7     What are the various energy transformations that occur when you are riding a bicycle?
        Sol.      While riding a bicycle, the muscular energy of the rider gets transferred into heat energy and kinetic energy of the bicycle. Heat energy heats the rider’s body. Kinetic energy provides a velocity to the bicycle. The transformation can be shown as: During the transformation, the total energy remains conserved.


        Q.8     Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
        Sol.       When we push a huge rock, there is no transfer of muscular energy to the stationary rock. Also, there is no loss of energy because muscular energy is transferred into heat energy, which causes our body to become hot.


        Q.9     A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
        Sol.     1 unit of energy is equal to 1 kilowatt hour (kWh).
                   1 unit = 1 kWh
                   1 kWh = 3.6 ×  106 J 
                   Therefore, 250 units of energy = 250 × 3.6 × 106  = 9 × 108J


        Q.11     What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer
        Sol.         Work is done whenever the given two conditions are satisfied:
        (i) A force acts on the body.
        (ii) There is a displacement of the body by the application of force in or opposite to the direction of force. If the direction of force is perpendicular to displacement, then the work done is zero.
        When a satellite moves around the Earth, then the direction of force of gravity on the satellite is
        perpendicular to its displacement. Hence, the work done on the satellite by the Ear this zero.


        Q.12     Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.
        Sol.          Yes. For a uniformly moving object. Suppose an object is moving with constant velocity. The net force acting on it is zero. But, there is a displacement along the motion of the object. Hence, there can be a displacement without a force.


        Q.13     A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
        Sol.         Work is done whenever the given two conditions are satisfied:
        (i) A force acts on the body.
        (ii) There is a displacement of the body by the application of force in or opposite to the direction of force. When a person holds a bundle of hay over his head, then there is no displacement in the bundle of hay.Although, force of gravity is acting on the bundle, the person is not applying any force on it. Hence, in the absence of force, work done by the person on the bundle is zero.


        Q.14     An electric heater is rated 1500 W. How much energy does it use in 10 hours?
        Sol.          Energy consumed by an electric heater can be obtained with the help of the expression,Where,
        Power rating of the heater, P = 1500 W = 1.5 kW
        Time for which the heater has operated, T = 10 h
        Work done = Energy consumed by the heater
        Therefore, energy consumed = Power × Time
        = 1.5 × 10 = 15 kWh
        Hence, the energy consumed by the heater in 10 h is 15 kWh.


        Q.15     Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
        Sol.         The law of conservation of energy states that energy can be neither created nor destroyed. It can only be  converted from one form to another. Consider the case of an oscillating pendulum. When a pendulum moves from its mean position P to either of its extreme positions A or B, it rises through a height h above the mean level P. At this point, the kinetic energy of the bob changes completely into potential energy. The kinetic energy becomes zero, and the bob possesses only potential energy. As it moves towards point P, its potential energy decreases progressively. Accordingly, thekinetic energy increases. As the bob reaches point P, its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates. The bob does not oscillate forever. It comes to rest because air resistance resists its motion. The pendulum loses its kinetic energy to overcome this friction and stops after some time. The law of conservation of energy is not violated because the energy lost by the pendulum to overcome friction is gained by its surroundings. Hence, the total energy of the pendulum and the surrounding system remain conserved.


        Q.16     An object of mass, m is moving with a constant velocity, v. How much work should be done on the object  in order to bring the object to rest?
        Sol.          Kinetic energy of an object of mass, m moving with a velocity, v is given by the expression,
        Hence, 20.8 × 104 J of work is required to stop the car.


        Q.18     In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
        Sol.        Work is done whenever the given two conditions are satisfied:
        (i) A force acts on the body.
        (ii) There is a displacement of the body by the application of force in or opposite to the direction of force. In this case, the direction of force acting on the block is perpendicular to the displacement. Therefore, work done by force on the block will be zero. In this case, the direction of force acting on the block is in the direction of displacement. Therefore, work done by force on the block will be positive. In this case, the direction of force acting on the block is opposite to the direction of displacement. Therefore, work done by force on the block will be negative.


        Q.19     Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
        Sol.          Acceleration in an object could be zero even when several forces are acting on it.
        This happens when all the forces cancel out each other i.e., the net force acting on the object is zero. For a uniformly moving object, the net force acting on the object is zero. Hence, the acceleration of the object is  zero. Hence, Soni is right.


        Q.20     Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.
        Sol.          Energy consumed by an electric device can be obtained with the help of the expression for power,
        Where,
        Power rating of the device, P = 500 W = 0.50 kW
        Time for which the device runs, T = 10 h
        Work done = Energy consumed by the device
        Therefore, energy consumed = Power × Time
        = 0.50 × 10 = 5 kWh
        Hence, the energy consumed by four equal rating devices in 10 h will be 4 × 5 kWh
        = 20 kWh = 20 Units.


        Q.21     A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
        Sol.          When an object falls freely towards the ground, its potential energy decreases and kinetic energy increases. As the object touches the ground, all its potential energy gets converted into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy. It can also deform the ground depending upon the nature of the ground and the amount of kinetic energy possessed by the object.

         

        Exemplar

        Multiple Choice Questions

        Q.1 When a body falls freely towards the earth, then its total energy

        (a) Increases

        (b) Decreases

        (c) remains constant

        (d) First increases and then decreases

        Sol. (c)


        Q.2 A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process the potential energy of the car

        (a) Does not change

        (b) Becomes twice to that of initial

        (c) Becomes 4 times that of initial

        (d) Becomes 16 times that of initial

        Sol. (a)


        Q.3 In case of negative work the angle between the force and displacement is

        (a) 0º

        (b) 45º

        (c) 90º

        (d ) 180º

        Sol. (d)


        Q.4 An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower. When they are 10 m above the ground, they have the same

        (a) Acceleration

        (b) Momenta

        (c) Potential energy

        (d) Kinetic energy

        Sol. (a)


        Q.5 A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g =10 m s–2)

        (a) 6 ×103 J

        (b) 6 J

        (c) 0.6 J

        (d) zero

        Sol. (d)


        Q.6 Which one of the following is not the unit of energy?

        (a) joule

        (b) Newton metre

        (c) Kilowatt

        (d) Kilowatt hour

        Sol. (c)


        Q.7 The work done on an object does not depend upon the

        (a) Displacement

        (b) Force applied

        (c) Angle between force and displacement

        (d) Initial velocity of the object

        Sol. d)


        Q.8 Water stored in a dam possesses

        (a) No energy

        (b) Electrical energy

        (c) Kinetic energy

        (d) Potential energy

        Sol. (d)


        Q.9 A body is falling from a height h. After it has fallen a height h 2 , it will possess

        (a) Only potential energy

        (b) Only kinetic energy

        (c) Half potential and half kinetic energy

        (d) More kinetic and less potential energy

        Sol. (c)

         

         

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