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1-MARK QUESTIONS Question.2 State the law of conservation of energy. [SAII-2014] More Resources for CBSE Class 9 Question.3 In a tug-of-war one team gives way to the other. What work is being done and by whom ? [SAII-2014] Question.4 What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity ? Question.5 List two essential conditions for work to be done. [SAII-2010] Question.6 When is 1 joule of work said to be done ? Question.7 What is the SI unit of work done and power ? Question.8 What is power? What is its SI unit ? Question.9 Find the energy in kWh consumed in 10 hours by a machine of power 500 W. [SAII-2011] Question.10. When is work said to be done against the force of gravity ? Question.11 Write an expression for the work done in lifting a body of mass ‘m’ through a vertical height ‘h’. [SAll-2012] Question.12 When a book is lifted from a table, against which force work is done ? Question.13 Will work be done by a man who pushes a wall ? Question.14 What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other ? Question.15 Is it possible that some force is acting on a body but still the work done is zero ? Question.16 What is the work done on a body moving in a circular path ? Question.17 Does every change in energy of the body involve work ? Question.18 What is the work done in the situation shown below ? Answer. Zero. Question.19 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 ? = 7 x 8 = 56 J Question.20 When do we say that work is done ? ~ Question.21 Write an expression for the work done when a force is acting on an object in the direction of its displacement. Question.22 A pair of bullocks exert 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 ? Question.23 What is the kinetic energy of an object? Question.24 Write an expression for the kinetic energy of an object. Question.25 Define 1 watt of power. Question.26 A lamp consumes 1000 J of electrical energy in 10 s. What is its power ? Using p = W/t = 1000/10 = 100 W Question.27 Define average power. Question.28 Define energy. Question.29 A body performs no work. Does it imply that the body possesses no energy ? Question.30 What is the SI unit of energy? Question.31 Does a body at rest possess any kinetic energy ? Question.32 What will happen to the kinetic energy of a body if its mass is doubled ? Question.33 What will happen to the kinetic energy of a body if its velocity is halved ? [SAII-2012] Question.34 By how much will the speed of a body, of fixed mass, increase if its kinetic energy becomes four times its initial kinetic energy ? Question.35 Can a body possess energy even if it is not in motion ? Question.36 Define potential energy. Question.37 Name the energy possessed by a stretched rubber band lying on the table. Question.38 Give the SI unit of potential energy. Question.39 What do you mean by trans- formation of energy ? Question.40 Can energy be destroyed? Can energy be created ? Question.41 A cell converts one form of energy into another. Name the two forms. Question.42 Name one unit of power bigger than watt. Question.43 When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy ? [SAll-2010] Question.44 Name at least three commonly used units of energy. Question.45 Name the practical unit of power in engineering. Question.46 Name at least six forms of energy. (vi)Solar energy Question.47 How many watt are there in 1 horse – power ? Question.48 What is horsepower ? Question.49 A light and a heavy body have equal kinetic energy. Which one is moving fast ? [SAII-2011] 2 MARKS QUESTIONS Question.2 Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example. [SAII-2012] Question.3 Define work. How is work measured ? When is work done by a force negative? [SAII-2013] Work done is negative if force and displacement are in the opposite direction. Question.4 What is the work done by the force of gravity in the following cases ? (b) Given m = 250 g = 0.25 kg, h = 2.5 m, g = 10 ms-2, W = ? Now, W = FS = mg x h = 0.25 x 10 x 2.5 = 6.25 J Question.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. Question.6 The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why? [SAII-2010] Question.7 What are the various energy transformations that occur when you are riding a bicycle? Question.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? Question.9 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 ? [SAll-2011 ] Question.10 A battery lights a bulb. Describe the energy changes involved in the process. Question.11 What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer. Question.12 Can there be displacement of an object in the absence of any force acting on it? Think; force. No, because force cannot be zero for displacement on the surface of earth. Some force is i essential. Question.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. Question.14 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 ? Question.15 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 ? Question.16 A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy ? [SAI1-2011] Question.17 What kinds of energy transformations take place at a thermal power station ? Question.18 Name the transformation of energy involved in the following cases : (e) Potential energy into kinetic energy. Question.19 What are the factors on which the work done depends ? [SAII-2010] (iii) The angle between force and displacement. Question.20 How are kinetic energy and momentum related ? Question.21 What is the work done by a coolie walking on a horizontal platform with a load on his head ? Question.22 We wind our watch once a day, what happens to the energy ? Question.23 What is the amount of work done by a force when a body moves in a circular path ? [SAll-2012] 3 MARKS QUESTIONS (vii)Work is done because displacement takes place in the direction of the force. Question.2 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 ? Question.3 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. Answer. Question.4 Distinguish between work, energy and power. State the SI units for each of these quantities. Question.5 A girl sits and stands repeatedly for 5 minutes. Draw a graph to show variation of potential energy with time. 5 MARKS QUESTIONS Question.2
Question.3 State the law of conservation of energy? Show that when a body falls from a certain height the total mechanical energy remains conserved. AT POINT B : Question.4 KE = 1/2 mv2, therefore, if velocity is made 5 times KE will increase by 25 times. (c)More the potential energy more will be the kinetic energy of the body when it falls. Hence, the body with mass 2m will have greater kinetic energy as it has more potential energy. Question.5 APPLICATION BASED QUESTIONSQuestion.1 A light body and a heavy body have the same kinetic energy. Which one will have the greater momentum ? Question.2 A light and a heavy body have the same momentum. Which one will have greater kinetic energy ? Question.3 Question.4 When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy ? Question.5 A spring which has been kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring ? Higher Order Thinking Skills (HOTS) QuestionsQuestion.1 justify giving proper reasoning whether the work done in the following cases is positive or negative : (e) Work done is negative because the resistive force of air always acts opposite of the direction of motion of the vibrating*pendulum. Question.2 What is the work done by a coolie walking on a horizontal platform with a load on his head? [SAII-2010] Question.3 The work done in lifting a box on to a platform does not depend upon how fast it is lifted up. Question.4 Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force ? Explain your answer giving a suitable example. Question.5 A body moves along a circular path. How much work is done in doing so? Explain. Question.6 A man rowing a boat upstream is at rest with respect to the shore. Is he doing work ? [SAII-2012] Question.7 What type of energy is stored in the spring of a watch? [SAII-2013] Question.8 What happens to the kinetic energy when : Question.9 When a constant force is applied to a body moving with constant acceleration, is the power of the force constant ? If not, how would force have to vary with speed for the power to be constant ? Question.10 A spring which is kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring ? Reasoning Questions
Question.2 (b) Yes, it is possible when force acts at right angles to the direction of motion of the body. Example Gravitational force of earth acts on a satellite at right angles to its direction of motion. Question.3 (b) Stretched string of a bow. Question.4 When is the work done by a force said to be negative? Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work. [SA II – 2012] Question.5
Important Topics/Areas/Questions which are frequently asked in the examinationQuestion.1 An object of mass mwhen raised To height h possess a potential energy of 1200 J. Find the new potential energy : Question.2 Define 1 watt of power. A lamp consumes 1000 ] of electrical energy in 10 s. Calculate its power. [SAII-2013] Question.3 Explain that the flying bird has; potential and kinetic energy and give their expressions. Question.4 Question.5 Question.6 Define : (a) power (b) work done (c) kinetic energy. Give SI unit of each. [SAII-2014] (c) It is the energy possessed by a body by virtue of its motion. Its SI unit is joule. Question.7 Define power. Write commercial unit and SI unit of electrical energy. An electrical geyser of 1.5 kW works for 2 hours. Find the electrical energy units consumed in a day. [SAII-2014] Question.8 The masses of scooter and bike are in the ratio of 2 : 3 but moving with same speed of 108 km h~1. Compute the ratio of their kinetic energy. [SAII-2014] NUMERICAL PROBLEMSQuestion.1 The kinetic energy of an object of mass ‘m’ moving with a velocity of 5 ms-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? Question.2 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. [SAII-2013] Question.3. A certain household has consumed 250 units of energy during a month. How much energy is this in joule? [SAU-2011] Question.4 An electric heater is rated 1500 W. How much energy does it use in 10 hours? Question.5 Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 kmh-1? [SAII-2010] Question.6 Find the energy in kWh consumed in 10 hours by four devices of power 500 W each. Question.7 Two bodies of equal masses move with the uniform velocities v and 3v respectively. Find the , ratio of their kinetic energies. [SAII-2013] Thus, the kinetic energy of the second body is nine times the kinetic energy of the first body. Question.8 A man of mass 60 kg runs up a flight of 30 steps in 40 seconds. If each step is 20 cm high, calculate the power of the man. [SAU-2012] Question.9 Calculate the amount of work done in moving a 50 kg block through a distance of 10 m by applying a force of 100 N. Question.10 A block of mass 5 kg is lying bn a frictionless table. A force of 20 N is applied on it for 10 seconds. Calculate its kinetic energy. Question.11 A girl of mass 40 kg climbs a rope 6 m long at constant speed in 15 seconds. What power she expands during the climb? Question.12 A man weighing 70 kg carries a weight of 10 kg to the top 6f a tower 100 m high. Calculate the work done. Question.13 Calculate the kinetic energy of a body of mass 2 kg moving with a velocity of 0.1 ms-1. Question.14 Find the velocity of a body of mass 100 g having a kinetic energy of 20 J. Question.15 A man drops a 10 kg rock from the top of a 5 m ladder. What is its kinetic energy when reaches the ground? What is its speed just before it hits the ground? Question.16 Which would have greater effect on kinetic energy of an object – doubling the mass, or doubling the velocity? Question.17 A body of mass 4 kg initially at rest is subjected to a force of I6 N. What is the kinetic energy acquired by the body at the end of 10 s? [SAII-2010] Question.18 A crane pulls up a car weighing 500 kg to a vertical height of 4 m. Calculate the work done by the crane. Hence, work done = FS = 4900 x 4 = 19600 J Question.19 A force of 10 N displaces a body by a distance of 2 m at an angle 60° to its own direction. Find the amount of work done. [SAII-2012] W = 10 x 2 x cos 60° = 10 x 2 x 1/2 =10J Question.20 A boy of mass 40 kg runs up flight of 50 steps each 10 cm high in 5 seconds. Find: (i) the work done by the boy. (ii) the power developed, (g = 9.8 ms-2) Question.21 Calculate the power of an engine required to lift 10s kg of coal per hour from a mine 360 m deep, (Take g = 10ms-2). Question.22 A man whose mass is 50 kg climbs up 30 steps of a stair in 30 s. If each step is 20 cm high, calculate the power used in climbing the stairs. [Take g = 10 ms-2] Question.23 A mass of 10 kg is dropped from a height of 50 cm. Find its : (i) Kinetic energy (ii) Velocity just as it reaches the ground. Does the velocity depend upon the mass of the particle? Explain. [Take g = 10 ms-2] [SAll-2012] Question.24 If you apply 1 J of energy to lift a book of 0.5 kg, how high will it rise? [Take g = 10 ms-2] Therefore, h = 0.2 m = 20 cm Question.25 A woman pulls a bucket of water of total mass 5 kg from a well which is 10 m in 10 s. Calculate the power used by her. [Take g = 10 ms-2] Question.26 |