Work, Energy, and Power PDF

Work 1) Work is done when a force produces motion. ((((Condition for a force to do work is that it should produce motion in an object e.g. a man may get completely exhausted in trying to push a stationary wall but since there is no displacement work done by the man on the wall is zero)))) Please not...

Work 1) Work is done when a force produces motion. ((((Condition for a force to do work is that it should produce motion in an object e.g. a man may get completely exhausted in trying to push a stationary wall but since there is no displacement work done by the man on the wall is zero)))) Please note work done on the body of the man himself is not zero because when the man pushes the ball his muscles are stretched and blood is displaced to the strained muscles more rapidly these changes consume energy and the man feels tired. 2) Work = force x distance Work done by a force on a body depends upon two factors a) magnitude of force and b) distance through which the body moves in the direction of force 3) SI unit of work = joule. 4) Work is a scalar quantity. Work has only magnitude and no direction. Q5)Define one joule of work. A5)When a force of 1N moves a body through a distance of 1 m in its own direction work done is said to be 1joule. 6) Whenever work is done against gravity work done is equal to product of weight of the body and the vertical distance through which the body is lifted. W = m x g x h 7) Sometimes the movement of the body is at an angle to the direction of the applied force. In such cases we can’t use the formula W = F x s to calculate the work done because distance moved is not exactly in the direction of force applied. In this case we use the formula W= F cos X s Where is the Angle between the direction of motion of the body and the direction of force applied. 8)Page no 131(new book-153) sample problem 1,2,3 & page no 134 (new book-156). 9) positive work a) work done is positive when a force acts in the direction of motion of the body. b) For example if we kick a football lying on the ground then the football starts moving in the direction of Our force. So the work done on the football by our leg is positive. Also note that in case of positive work the force increases the speed of the body. 10) Negative work a) When the force acts opposite to the Direction of motion of a body then the work done is negative. b) Football moving on the ground slows down gradually and ultimately stops this is because of force of friction. Force of friction acts in the direction opposite to the direction of football so in this case work done by the force of friction is negative. Please note that the negative work decreases the speed of the body. c) When a boy throws a ball vertically upwards then the force applied by the boy on the ball does positive work but the gravitational force of earth acting on the upward going ball does the negative work. d) In this case angle between the direction of motion and direction of force is 180 degrees. W= F cos 180 x s = — F x s. ( cos 180 = —1). 11 zero work done When force on the object is zero then work done is zero. When the displacement of the object is zero, then work done is zero. a) Work done is zero when a force acts at right angles to the direction of motion of the body. Angle between the direction of motion and direction of force is 90° W = F cos 90 x s = zero. ( cos 90 = 0 ) So when the displacement of body is perpendicular to the direction of force no work is done. b) E.g a man carrying a suitcase on his head does no work with respect to gravity. because force of gravity acts vertically downwards and his displacement is perpendicular to the force of gravity. c) To keep a body moving in a circle there must be a force acting on it directed towards the centre of the circle. This force is called centripetal force. This centripetal force acts along the radius of the circle at right angles to the motion of the body. d) Work done by earth moving around the sun is zero because gravitational force acts at right angles to the direction of motion of earth. e) Work done by satellite moving around the earth is also zero because gravitational force acts at right angles to the direction of motion of satellite. f) Zero work done does not effect speed of the body.⭐ ⭐ ⭐ Energy 1) Energy is the ability to do work. 2)The Amount of energy possessed by a body is equal to the amount of work it can do when the energy is released. 3) Energy is a scalar quantity. It has only magnitude but no direction. 4) SI unit of energy is Joules. 1 kJ = 1000 joules. 5) Different forms of energy kinetic energy, chemical energy, heat energy, potential energy, light energy and sound energy. 6) Energy of a body due to its motion is called Kinetic energy. 7) A moving body is capable of doing work. For example A moving bullet can penetrate even a steel plate due to its KE. A running motorcycle has KE, a running car has KE, a moving hammer has KE. when moving body is brought to rest by the opposing force KE is used up (or lost ) in overcoming the resistance of opposing force and hence the body comes to rest. 8) Kinetic energy of a moving body is made by the amount of work done it can do before coming to rest. KE = 1mv2 9) From this we can see KE depends upon two things a) KE of a body is directly proportional to the mass of the body. b) KE of a body is directly proportional to the square of velocity of the body. ******If the mass of the body is doubled KE also gets doubled and if the mass of the body is halved it’s KE also gets halved. ******If the velocity of a body is doubled its KE becomes four times and if velocity of the body is halved it’s KE becomes 1/4. *******Doubling the velocity has greater effect on its KE then doubling the mass. ********Since KE of a body depends on its mass and velocity therefore heavy bodies moving with high velocity have more KE then slow bodies of same mass. This is the reason Why blacksmith uses a heavier hammer then one used by Goldsmith. Q10) While a driver increases the speed of his car on approaching a hilly Road? A10) When the car is going uphill in addition to friction and air resistance it has to do work against force of gravity as well. A driver increases the speed of car on approaching the hilly road to give more kinetic energy to the car. 11) Energy of a body due to its position above the ground is called gravitational potential energy.brick lying on the roof. 12) The energy of the body due to change in its shape and size is called elastic potential energy. Elastic potential energy is associated with the state of compression or extension of an object, for example, compressed spring/extended spring/compressing/stretching/bending/ twisting/wound up circular spring in toys/ stretched strings in catapult/ stretched strings in bow and arrow/ 13) Suppose a body of mass m is raised to a height h above the surface of the earth. The force acting on the body is the gravitational pull of the earth = mg acting in the downward direction. work done against this force of gravity in lifting a body is equal to its potential energy. Work done = Force x Distance (F=mg why ?) W = mgh = Potential energy Sample problem 1,2&3 on page no 141/ new book-163. Sample problem 1&2 on page no 144 / new book-167 14) A body can have both potential energy as well as kinetic energy. Sum of potential energy and kinetic energy is called its mechanical energy. A flying bird, a flying aeroplane, man climbing a hill has kinetic energy as well as potential energy. 15) A stone lying on the top of a hill has only potential energy. When this stone starts rolling downwards, it has both kinetic and potential energy. And when this stone reaches the bottom of the hill it has only kinetic energy. Power Q1) Define power A1) Power is defined as the rate of doing work. Power = Work done / Time taken 2) When work is done equal amount of energy is consumed. Q3) Define 1 Watt. A3) One watt is the power of an appliance which does work at the rate of 1 Joule per second. 4) 1 kW = 1000 watt 5) 1 MW = 1000000 watt 6) 1 hp = 746 watt 7) Power is a scalar quantity. 8) The more powerful a car is quicker it will do the work (climb a steep road). 9) The more powerful a machine is more rapidly it can do work. Q10) Sample problem 1,2,3 on page number 152/ new book-176 Q11) Why filament type bulb are not energy-efficient? A11) Filament type lamps produces light but also waste a lot of electrical energy in form of heat hence they are not energy efficient. Hence these days we use CFL ( compact fluorescent lamps) Q12) What is commercial unit of electrical energy or energy? A12) The commercial unit of energy is kilowatt hour (kW-h) ((((electrical energy used in homes shops and industries is measured in kWh)))) (((((Joule represents a very small quantity of energy hence it is inconvenient to use it as commercial unit of energy))))). Q13) Define 1 kWh. A13) 1 kWh is the amount of electrical energy consumed when an electrical appliance having a power of 1 kW is used for 1hour. Q14) Write the relationship between kilowatt hour and Joule A14) 1kWh = 3.6 x 106 joules Q15) What is one unit of electrical energy? A15) 1 kWh of electrical energy is commonly known as one unit of electricity 16) Page number 154 (new book-179) sample problems 1&2. Transformation of energy Q1) What is transformation of energy? A1) The change of one form of energy into another form of energy is known as transformation of energy. Q2)Give an example of transformation of energy a) When a body is released from a height then the potential energy of the body is gradually transformed into kinetic energy. Suppose a stone is lying on the roof of a house. All the energy of the stone is in form of potential energy. When the stone is dropped from the roof its P.E starts changing into K.E It’s P.E goes on decreasing ( Because its height is decreasing) but it’s K.E goes on increasing (because it’s velocity goes on increasing).When the stone just reaches the ground it’s P.E becomes zero at this stage and we can say all P.E has been converted into K.E. b) When a body is thrown upwards the kinetic energy of the body is gradually transformed into potential energy. When a stone is thrown upwards the initial KE of the stone starts changing into PE. As the stones moves up its KE goes on decreasing (because it’s velocity goes on decreasing) but it’s PE goes on increasing (because it’s height goes on increasing). In other words KE of the stone gradually gets transformed into PE and when the stone reaches at the highest point KE becomes zero and it’s PE becomes maximum at the highest point And we can say that all KE has been converted into PE. Q3)energy conversions at : a) Hydroelectric power house Potential energy -> kinetic energy ->electrical energy b) Thermal power House Chemical energy -> heat energy -> kinetic energy -> electrical energy c) Electric motor Electrical energy -> mechanical energy d) Electric iron electric energy -> heat energy e) Electric bulb electric energy -> heat energy -> light energy f) Radio electrical Energy -> kinetic energy -> sound energy g) Steam engine heat energy -> kinetic energy h) Car engine chemical energy -> heat energy -> kinetic energy i) Cell or battery chemical energy -> electrical energy j) Gas stove chemical energy -> heat energy k) Solar water heater light energy -> heat energy l) Solar cell light energy -> electrical energy Q4) Write Law of conservation of energy. A4) Energy can neither be created nor destroyed. ((((during conversion of energy from one form to another some energy may be wasted but the total energy of the system remains same )))) 5) Sum of potential energy & kinetic energy of the body falling from a height remains the same at every point during its fall. Total energy of the swinging pendulum at any instant of time remains constant (conserved) 6) Example of conservation of energy a) For a stone lying on the roof of a house, all the energy of the stone is in form of potential energy. When the stone is dropped from the roof its P.E starts changing into K.E It’s P.E goes on decreasing ( because height is decreasing) but it’s K.E goes on increasing (because it’s velocity goes on increasing). When the stone just reaches the ground it’s P.E becomes zero at this stage and we can say all P.E has been converted into K.E. There is no destruction of energy this is an example of conservation of energy during the free fall. b) When a stone is thrown upwards the initial KE of the stone starts changing into PE. As the stones moves up its KE goes on decreasing (because it’s velocity goes on decreasing) but it’s PE goes on increasing (because it’s height goes on increasing). In other words KE of the stone gradually gets transformed into PE and when the stone reaches at the highest point KE becomes zero and it’s PE becomes maximum at the highest point And we can say that all KE has been converted into PE. There is no destruction of energy this is an example of conservation of energy. 7) Body which does work loses energy & the body on which work is done gains energy. 8) page no 161(new book 186) pendulum diagram (figure 66 schand) Q) IMP QUES : Two girls each of weight, 400N climb up a rope through a height of 8 m. We name one of the girls A and the other B. A takes 20s while B takes 50s to accomplish this task. What is the power expended by each girl? A) A =160W; B=64W. Q) Express the kinetic energy of an object in the form of an equation. A) Consider an object of mass, m moving with a uniform velocity, u. Let it now be displaced through a distance s when a constant force, F acts on it in the direction of its displacement. the work done, W= F s ———————- eq1 The work done on the object will cause a change in its velocity. Let its velocity change from u to v. Let a be the acceleration produced. We know v2 - u2 = 2as This gives s = v2 - u2 /2a ————— eq2 We know F = m.a. ———————— eq3 Put eq2 and eq3 in eq1 W= ma x (v2 - u2 /2a) W= 1/2 [m(v2 - u2 )] If the object is starting from rest then u= 0 W =1/2 [m(v2)] Thus K.E possessed by an object of mass m and moving with a uniform velocity v is Ek = 1/2 [m(v2)]