Work And Energy

Information about Work And Energy

Published on January 25, 2011

Author: sambhukannan



Slide 1: ASWIN SAMBHU.P.R CLASS:IX.A Presents..... PHYSICS PROJECT Slide 2: Objective: WORK & ENERGY Slide 3: WORK We all are familiar with the word ‘work’. We do a lot of work everyday. But in science ‘WORK’ has another meaning. According to science, a work is said to be done only when a force act on an object which displaces it or which causes the object to move. Therefore the two conditions required to prove that a work is done are : A force should act on an object. The object must be displaced. If any one of the above conditions does not exist, then work is not done. WORK is a scalar quantity, i.e. it has only magnitude and no direction. The unit of WORK is Neuton metre (N m) or joule (J). Slide 4: Arya pushed a large piece of rock and the rock moved through a distance. Akhil pulled a box and the table moved through a distance. Megha kicked a football and the ball moved a little. Ashish tried to pushed a refrigirator in his room, but it did not move. Anandu tried to lift a bench lying on the floor, but it did not move. Harsha kicked a tank full of water, but it did not get displaced. SOME EXAMPLES WORK is done WORK is not done Here, WORK is done because the applied force displaced the object or cause the object to move. Here, WORK is not done because the applied force could not move the object or cause displacement. Slide 5: Let a constant force, F act on an object. Let the object be displaced through a distance, s in the direction of the force. Let W be the work done. So we define work to be equal to the product of the force and displacement. → Work done = Force x Displacement Work Done By A Constant Force W = Fs For Example : If F=1N and s=1m then the work done by the force will be 1Nm. Slide 6: WORK done by a constant force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. If the force and the displacement are in the same direction, then the WORK done will be equal to the product of the force and displacement i.e. the WORK done will be positive. W=Fs. If the force acts opposite to the direction of displacement, then the WORK done will be negative i.e. W=F x (-s) or (-F x s). Slide 7: A force of 7N acts on an object. The displacement is, say 8m, 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? The applied = 7N Its displacement = 8m Work done = Force x displacement = 7N x 8m = 56 N m = 56J. Therefore the work done is 56J. QUESTIONS Slide 8: A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5m above the ground. Calculate the work done by him on the luggage. Mass of luggage = 15kg Its displacement = 1.5m Work done,W = F x s = mg x s = 15 kg x 10 m sˉ² x 1.5m = 225 N m = 225J. Therefore the work done is 225J. Define 1 J of work. 1 J of work is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force. Slide 9: ENERGY is the capability of doing work. An object having the capability to do work is said to posses energy. The object which does the work loses energy and the object on which the work is done gains energy. An object that possesses energy can exert a force on another object. When this happens, energy is transferred from the former to the later. The second object may move as it receives energy and therefore do some work. Any object that possesses energy can do work. The unit of energy is the same as that of work. i.e. joule (J) Energy Slide 10: When a fast moving ball hits a stationary wicket, the wicket is thrown away. When a raised hammer falls on a nail placed on a piece of wood, it drives the nail into the wood. When an air filled balloon is pressed, it will change its shape. If we press the balloon hard it will explode producing a blasting sound. SOME EXAMPLES Slide 11: Forms Of Energy We have many different forms of ENERGY. The various forms include: Mechanical Energy (Potential Energy + Kinetic Energy) Heat Energy Chemical Energy Electrical Energy Light Energy Slide 12: James Prescott Joule James Prescott Joule ( 24 December 1818 – 11 October 1889) was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The SI derived unit of energy, the joule, is named after him. He worked with Lord Kelvin to develop the absolute scale of temperature, made observations onmagnetostriction, and found the relationship between the current through a resistance and the heat dissipated, now called Joule's law. Slide 13: The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of the given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work would be done by the body in decelerating from its current speed to a state of rest. The speed, and thus the kinetic energy of a single object is completely frame-dependent (relative): it can take any non-negative value, by choosing a suitable inertial frame of reference. For example, a bullet racing past an observer has kinetic energy in the reference frame of this observer, but the same bullet is stationery, and so has zero kinetic energy, from the point of view of an observer moving with the same velocity as the bullet. Kinetic Energy Slide 14: By contrast, the total kinetic energy of a system of objects cannot be reduced to zero by a suitable choice of the inertial reference frame, unless all the objects have the same velocity. In any other case the total kinetic energy has a non-zero minimum, as no inertial reference frame can be chosen in which all the objects are stationery. This minimum kinetic energy contributes to the system's invariant mass, which is independent of the reference frame. According to classical mechanics (i.e. ignoring relativistic effects) the kinetic energy of a non-rotating object of mass m traveling at a speed v is mv2/2. This will be a good approximation provided v is much less than the speed of light. Slide 15: Potential Energy Potential energy is energy that is stored within a system. It exists when there is a force that tends to pull an object back towards some lower energy position. This force is often called a restoring force. For example, when a spring is stretched to the left, it exerts a force to the right so as to return to its original, unstretched position. Similarly, when a mass is lifted up, the force of gravity will act so as to bring it back down. The action of stretching the spring or lifting the mass requires energy to perform. The energy that went into lifting up the mass is stored in its position in the gravitational field, while similarly, the energy it took to stretch the spring is stored in the metal. According to the law of conservation of energy, energy cannot be created or destroyed; hence this energy cannot disappear. Instead, it is stored as potential energy. If the spring is released or the mass is dropped, this stored energy will be converted into kinetic energy by the restoring force, which is elasticity in the case of the spring, and gravity in the case of the mass. Think of a roller coaster. When the coaster climbs a hill it has potential energy. At the very top of the hill is its maximum potential energy. When the car speeds down the hill potential energy turns into kinetic. Kinetic energy is greatest at the bottom. Slide 16: The more formal definition is that potential energy is the energy difference between the energy of an object in a given position and its energy at a reference position. There are various types of potential energy, each associated with a particular type of force. More specifically, every conservative force gives rise to potential energy. For example, the work of an elastic force is called elastic potential energy; work of the gravitational force is called gravitational potential energy; work of the Coulomb force is called electric potential energy; work of the strong nuclear force or weak nuclear force acting on the baryon charge is called nuclear potential energy; work of intermolecular forces is called intermolecular potential energy. Chemical potential energy, such as the energy stored in fossil fuels, is the work of the Coulomb force during rearrangement of mutual positions of electrons and nuclei in atoms and molecules. Thermal energy usually has two components: the kinetic energy of random motions of particles and the potential energy of their mutual positions. As a general rule, the work done by a conservative force F will be W = -ΔU where ΔU is the change in the potential energy associated with that particular force. Common notations for potential energy are U, Ep, and PE. Slide 17: An object increases its energy when raised throughout a height. This is because work is done on it against gravity while it is being raised. The energy present in such an object is the gravitational potential energy. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity. It is easy to arrive at an expression for the gravitational potential energy of an object at a height. Consider an object of mass m. Let it be raised through a height, h from the ground. A force is required to do his. The minimum force is required to raise the object is equal to the weight of the object, mg. The object gains energy equal to the work done on it. Let the work done on the object against gravity be W. That is Work done = Force x Displacement = mg x h = mgh Since work is done on the object is equal to mgh, an energy equal to mgh unit is gained by the object. This is the potential energy (E ) of the object. E = mgh It is useful to note that work done by gravity depends on the difference in vertical heights of the initial and final positions of the object and not on the path along which the object is moved. Potential Energy Of An Object At A Height p Slide 18: All of us do not work at the same rate. All machines do not consume or transfer energy at the same rate. Agents that transfer energy do work at different rates. A stronger person may do certain work in relatively less time. A more powerful vehicle would complete a journey in a shorter time than a less powerful one. We talk of the power of motorbikes and motorcars. The speed with which these vehicles change energy or do work is a basis for their classification. Power measures the speed of work done, i.e. how fast or slow work is done. Power is defined as the rate of transfer of energy. If an agent does a work W in time t, then power is given by: Power = Work/Time i.e. P = W/t Rate Of Doing Work Slide 19:  The unit of POWER is watt [in honour of James Watt (1736-1819)] having the symbol W. 1 watt is the power of an agent, which does work at the rate of 1 joule per second. 1 watt = 1 joule/second or 1W = 1 J sˉ¹. We express larger rates of energy transfer in kilowatts (kW). 1 Kilowatt = 1000 watts 1 kW = 1000 W 1 kW = 1000 J sˉ¹ The power of an agent may vary with time. This means that the agent may be doing work at different rates at different intervals of time. Therefore the concept of average power is useful. We obtain average power by dividing the total energy consumed by the total time taken. Average power = Total energy consumed/Total time taken Slide 20: Law Of Conservation Of Energy The law of conservation of energy states that energy cannot be created or destroyed., and that neither one appears without the other. Thus in closed systems, both mass and energy are conserved separately, just as was understood in pre-relativistic physics. The new feature of relativistic physics is that "matter" particles (such as those constituting atoms) could be converted to non-matter forms of energy, such as light; or kinetic and potential energy (example: heat). However, this conversion does not affect the total mass of systems, since the latter forms of non-matter energy still retain their mass through any such conversion. Slide 21: Today, conservation of “energy” refers to the conservation of the total system energy over time. This energy includes the energy associated with the rest mass of particles and all other forms of energy in the system. In addition, the invariant mass of systems of particles (the mass of the system as seen in its center of mass inertial frame, such as the frame in which it would need to be weighed) is also conserved over time for any single observer, and (unlike the total energy) is the same value for all observers. Therefore, in an isolated system, although matter (particles with rest mass) and "pure energy" (heat and light) can be converted to one another, both the total amount of energy and the total amount of mass of such systems remain constant over time, as seen by any single observer. If energy in any form is allowed to escape such systems (see binding energy), the mass of the system will decrease in correspondence with the loss. A consequence of the law of energy conservation is that perpetual motion machines can only work perpetually if they deliver no energy to their surroundings. Slide 22:  The unit joule is too small and is inconvenient to express large quantities of energy. We use a bigger unit called kilowatt hour (kW h). For example, we have a machine that uses 1000 J of energy every second. If this machine is used continues for an hour, it will consume 1 kW h of energy. Thus 1 kW h of energy is the energy used in one hour at the rate of 1000 J sˉ¹ (or 1 kW). 1 kW h = 1 kW x 1 h = 1000 W x 3600 s = 3600000 J 1 kW h = 3.6 x 10⁶ J. The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of ‘units’. Here 1 unit means 1kilowatt hour. Slide 23: What is the kinetic energy of an object? The kinetic energy of an object is the energy which it possesses due to its motion. An object of mass 15 kg is moving with a uniform velocity of 4 m sˉ². What is the kinetic energy possessed by the object? Mass of the object, m = 15 kg Velocity of the object = 4 m sˉ¹ → E = ½ m v² = ½ x 15 kg x 4 m sˉ¹ = 120J The kinetic energy of the object is 120 J. QUESTIONS k Slide 24: What is the work to done to increase the velocity of a car from 30 km hˉ¹ to 60 km hˉ¹ if the mass of the car is 1500 kg ? Mass of car, m = 1500 kg Initial velocity of the car, u = 30 km hˉ¹ = 30 x 1000m 60 x 60s = 8.33 m sˉ¹ . Similarly the final velocity of the car, v = 60 km hˉ¹ = 16.67 m sˉ¹. Therefore, the initial kinetic energy of the car. E = ½ m u² = ½ x 1500 kg x (8.33 m sˉ¹)² = 52041.68 J The final kinetic energy of the car, E = ½ x 1500 kg x (16.67 m sˉ¹)² = 208416.68 J Thus, the work done = Change in kinetic energy = E - E = 156375 J ki kf ki kf Slide 25: Find the energy possessed by an object of mass 10 kg when it is at a height of 6 m above the ground. Given, g = 9.8 m sˉ². Mass of object, m = 10 kg Its displacement (height), h = 6 m Acceleration due to gravity, g = 9.8 m sˉ² Potential energy = mgh = 10 kg x 9.8 m sˉ² x 6 m = 588 J. The potential energy is 588 J. An object of mass 12 kg is at a certain height above the ground. If the potential energy of the object is 480 J, find the height at which the object is with respect to the ground. Given, mg = 10 m sˉ². Mass of the object, m = 12 kg Potential energy, E = 480 J. E = mgh 489 J = 12 kg x 10 m sˉ² x h h = 480J = 4m 120 kg msˉ² The object is at the height of 4 m. p p Slide 26: Two girls, each of weight 400 N climb up a ro[e through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl ? (i) Power expended by girl A: Weight of the girl, mg = 400 N Displacement (height), h = 8 m Time taken, t = 20 s Power, P = Work done/Time taken = mgh/t = 400N x 8 m 20 s = 160 W. (ii) Power expended by girl B: Weight of the girl, mg = 400 N Displacement (height), h = 8 m Time taken, t = 50 s Power, P = mgh/t = 400 N x 8 m 50 s = 64 W Power expended by girl A is 160 W. Power expended by girl B is 64 W. Slide 27: A boy of mass 50 kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15 cm, find his power. Take g = 10 m sˉ². Weight of the boy, mg = 50 kg x 10 m sˉ² = 500 N Height of the staircase, h = 45 x 15/100 m = 6.75 m Time taken to climb, t = 9 s Power, P = Work done/Time taken = mgh/t = 500 N x 6.75 m 9 s = 375 W. Power is 375 W. An electric bulb of 60 W is used for 6 h per day. Calculate the ‘units’ of energy consumed in one day by the bulb. Power of electric bulb = 60 W = 0.06 kW. Time used, t = 6 h Energy = Power x Time taken = 0.06 kW x 6 h = 0.36 kW h = 0.36 ‘units’. The energy consumed by the bulb is 0.36 ‘units’. Slide 28: THANK YOU By : Aswin Sambhu.P.R

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