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# Work and energy physics 9 class

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### Work and energy physics 9 class

1. 1. We all are familiar with the word ‘work’. We do a lot of workeveryday. But in science ‘WORK’ has another meaning.According to science, a work is said to be done only when a force act onan 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).
2. 2. Arya pushed a large piece of Ashish tried to pushed a rock and the rock moved refrigirator in his room, but it did through a distance. not move. Akhil pulled a box and the Anandu tried to lift a bench lying table moved through a on the floor, but it did not move. distance. Megha kicked a football and Harsha kicked a tank full of the ball moved a little. water, but it did not get displaced. Here, W ORK is done because the Here, W ORK is not done becauseapplied force displaced the object or the applied force could not movecause the object to move. the object or cause displacement.
3. 3. 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 W ==F s s W FFor Example : If F =1N and s =1m then the work done by the force will be1Nm .
4. 4. WORK done by a constant force acting on an object isequal to the magnitude of the force multiplied by thedistance moved in the direction of the force. If the force and the displacement are in the samedirection, then the WORK done will be equal to theproduct of the force and displacement i.e. the WORKdone will be positive. W=Fs. If the force acts opposite to the direction ofdisplacement, then the WORK done will be negative i.e.W=F x (-s) or (-F x s).
5. 5. A force of 7N acts on an object. The displacement is, say 8m, in thedirection of the force. Let us take it that the force acts on the objectthrough the displacement. What is the work done in this case?The applied = 7NIts displacement = 8mWork done = Force x displacement = 7N x 8m = 56 N m = 56J.Therefore the work done is 56J.
6. 6. A porter lifts a luggage of 15 kg from the ground and puts it on his head1.5m above the ground. Calculate the work done by him on the luggage.Mass of luggage = 15kgIts displacement = 1.5mWork 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 Ndisplaces it by 1 m along the line of action of the force.
7. 7. 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 whichthe 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 somework. Any object that possesses energy can do work. The unit of energy is the same as that of work. i.e. joule (J)
8. 8. When a fast moving ball hits a stationary wicket, the wicketis thrown away.When a raised hammer falls on a nail placed on a piece ofwood, it drives the nail into the wood.When an air filled balloon is pressed, it will change itsshape. If we press the balloon hard it will explodeproducing a blasting sound.
9. 9. We have many different forms of ENERGY. The various formsinclude:Mechanical Energy (Potential Energy + Kinetic Energy)Heat EnergyChemical EnergyElectrical EnergyLight Energy
10. 10. James Prescott Joule ( 24 December 1818 – 11 October 1889) was anEnglish physicist and brewer, born in Salford, Lancashire. Joule studied thenature of heat, and discovered its relationship to mechanicalwork (see energy). This led to the theory of conservation of energy, which ledto the development of the first law of thermodynamics. The SI derived unit ofenergy, the joule, is named after him. He worked with Lord Kelvin to developthe absolute scale of temperature, made observations onmagnetostriction, andfound the relationship between the current through a resistance and the heatdissipated, now called Joules law.
11. 11. The kinetic energy of an object is the energy which it possesses due toits motion. It is defined as the work needed to accelerate a body of thegiven mass from rest to its current velocity. Having gained this energyduring its acceleration, the body maintains this kinetic energy unless itsspeed changes. The same amount of work would be done by the body indecelerating from its current speed to a state of rest. The speed, and thus the kinetic energy of a single object is completelyframe-dependent (relative): it can take any non-negative value, by choosinga suitable inertial frame of reference. For example, a bullet racing past anobserver has kinetic energy in the reference frame of this observer, but thesame bullet is stationery, and so has zero kinetic energy, from the point ofview of an observer moving with the same velocity as the bullet.
12. 12. By contrast, the total kinetic energy of a system of objectscannot be reduced to zero by a suitable choice of the inertialreference frame, unless all the objects have the same velocity. Inany other case the total kinetic energy has a non-zero minimum,as no inertial reference frame can be chosen in which all theobjects are stationery. This minimum kinetic energy contributesto the systems invariant mass, which is independent of thereference frame. According to classical mechanics (i.e. ignoring relativisticeffects) the kinetic energy of a non-rotating objectof mass m traveling at a speed v is mv2/2. This will be a goodapproximation provided v is much less than the speed of light.
13. 13.  Potential energy is energy that is stored within a system. It exists when there isa force that tends to pull an object back towards some lower energy position. Thisforce is often called a restoring force. For example, when a spring is stretched to theleft, 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 backdown. The action of stretching the spring or lifting the mass requires energy toperform. The energy that went into lifting up the mass is stored in its position inthe gravitational field, while similarly, the energy it took to stretch the spring isstored in the metal. According to the law of conservation of energy, energy cannot becreated or destroyed; hence this energy cannot disappear. Instead, it is stored aspotential energy. If the spring is released or the mass is dropped, this stored energywill be converted into kinetic energy by the restoring force, which is elasticity in thecase of the spring, and gravity in the case of the mass. Think of a roller coaster. Whenthe coaster climbs a hill it has potential energy. At the very top of the hill is itsmaximum potential energy. When the car speeds down the hill potential energy turnsinto kinetic. Kinetic energy is greatest at the bottom.
14. 14. The more formal definition is that potential energy is the energy differencebetween the energy of an object in a given position and its energy at areference position. There are various types of potential energy, each associated with aparticular type of force. More specifically, every conservative force gives riseto potential energy. For example, the work of an elastic force is called elasticpotential energy; work of the gravitational force is called gravitationalpotential energy; work of the Coulomb force is called electric potentialenergy; work of the strong nuclear force or weak nuclear force acting onthe baryon charge is called nuclear potential energy; work of intermolecularforces is called intermolecular potential energy. Chemical potential energy,such as the energy stored in fossil fuels, is the work of the Coulomb forceduring rearrangement of mutual positions of electrons and nuclei in atomsand molecules. Thermal energy usually has two components: the kineticenergy of random motions of particles and the potential energy of theirmutual positions. As a general rule, the work done by a conservative force F will be W = -ΔUwhere ΔU is the change in the potential energy associated with thatparticular force. Common notations for potential energy are U, Ep, and PE.
15. 15. An object increases its energy when raised throughout a height. This is because workis done on it against gravity while it is being raised. The energy present in such anobject is the gravitational potential energy. The gravitational potential energy of an object at a point above the ground is definedas 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 objectat a height. Consider an object of mass m. Let it be raised through a height, h from the ground. Aforce is required to do his. The minimum force is required to raise the object is equal tothe weight of the object, mg. The object gains energy equal to the work done on it. Letthe 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 gainedby the object. This is the potential energy (E ) of the object. Ep = mgh It is useful to note that work done by gravity depends on the difference in verticalheights of the initial and final positions of the object and not on the path along whichthe object is moved.
16. 16. All of us do not work at the same rate. All machines do notconsume or transfer energy at the same rate. Agents that transferenergy do work at different rates.A stronger person may do certain work in relatively less time. Amore powerful vehicle would complete a journey in a shortertime than a less powerful one. We talk of the power of motorbikesand motorcars. The speed with which these vehicles changeenergy or do work is a basis for their classification. Powermeasures the speed of work done, i.e. how fast or slow work isdone. Power is defined as the rate of transfer of energy. If anagent does a work W in time t, then power is given by: Power = Work/Time i.e. P = W/t
17. 17. 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 atthe 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 agentmay be doing work at different rates at different intervals of time.Therefore the concept of average power is useful. We obtain average powerby dividing the total energy consumed by the total time taken. Average power = Total energy consumed/Total time taken
18. 18. The law of conservation of energy states that energy cannot be createdor destroyed., and that neither one appears without the other. Thus inclosed systems, both mass and energy are conserved separately, just aswas understood in pre-relativistic physics. The new feature of relativisticphysics is that "matter" particles (such as those constituting atoms) couldbe converted to non-matter forms of energy, such as light; or kinetic andpotential energy (example: heat). However, this conversiondoes not affect the total mass of systems, since the latter forms of non-matter energy still retain their mass through any such conversion.
19. 19. Today, conservation of “energy” refers to the conservation of the total systemenergy over time. This energy includes the energy associated with the rest mass ofparticles and all other forms of energy in the system. In addition, the invariantmass of systems of particles (the mass of the system as seen in its center ofmass inertial frame, such as the frame in which it would need to be weighed) isalso conserved over time for any single observer, and (unlike the total energy) isthe 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 toone another, both the total amount of energy and the total amount of mass of suchsystems remain constant over time, as seen by any single observer. If energy in anyform is allowed to escape such systems (see binding energy), the mass of thesystem will decrease in correspondence with the loss. A consequence of the law of energy conservation is that perpetualmotion machines can only work perpetually if they deliver no energy to theirsurroundings.
20. 20. The unit joule is too small and is inconvenient to express large quantitiesof 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 machineis used continues for an hour, it will consume 1 kW h of energy. Thus 1 kWh 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 establishmentsare usually expressed in kilowatt hour. For example, electrical energy usedduring a month is expressed in terms of ‘units’. Here 1 unit means1kilowatt hour.
21. 21. What is the kinetic energy of an object?The kinetic energy of an object is the energy which it possesses due toits 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 kgVelocity of the object = 4 m sˉ¹ → E k = ½ m v² = ½ x 15 kg x 4 m sˉ¹ = 120JThe kinetic energy of the object is 120 J.
22. 22. What is the work to done to increase the velocity of a car from 30 km hˉ¹ to60 km hˉ¹ if the mass of the car is 1500 kg ?Mass of car, m = 1500 kgInitial 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 ki = ½ m u² = ½ x 1500 kg x (8.33 m sˉ¹)² = 52041.68 JThe final kinetic energy of the car, Ekf = ½ x 1500 kg x (16.67 m sˉ¹)² = 208416.68 JThus, the work done =kf Change in kinetic energy ki =E -E
23. 23. Find the energy possessed by an object of mass 10 kg when it is at aheight of 6 m above the ground. Given, g = 9.8 m sˉ².Mass of object, m = 10 kgIts displacement (height), h = 6 mAcceleration due to gravity, g = 9.8 m sˉ²Potential 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 thepotential energy of the object is 480 J, find the height at which the objectis with respect to the ground. Given, mg = 10 m sˉ².Mass of the object, m = 12 kgPotential energy, E p = 480 J. E p = mgh 489 J = 12 kg x 10 m sˉ² x h h= 480J = 4m 120 kg msˉ²The object is at the height of 4 m.
24. 24. Two girls, each of weight 400 N climb up a ro[e through a height of 8 m. Wename one of the girls A and the other B. Girl A takes 20 s while B takes 50 sto accomplish this task. What is the power expended by each girl ?(i) Power expended by girl A:Weight of the girl, mg = 400 NDisplacement (height), h = 8 mTime taken, t = 20 sPower, 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 NDisplacement (height), h = 8 mTime taken, t = 50 sPower, P = mgh/t = 400 N x 8 m 50 s = 64 WPower expended by girl A is 160 W.Power expended by girl B is 64 W.
25. 25. A boy of mass 50 kg runs up a staircase of 45 steps in 9 s. If the height ofeach step is 15 cm, find his power. Take g = 10 m sˉ².Weight of the boy, mg = 50 kg x 10 m sˉ² = 500 NHeight of the staircase, h = 45 x 15/100 m = 6.75 mTime taken to climb, t = 9 sPower, P = Work done/Time taken = mgh/t = 500 N x 6.75 m 9s = 375 W.Power is 375 W.An electric bulb of 60 W is used for 6 h per day. Calculate the ‘units’ ofenergy consumed in one day by the bulb.Power of electric bulb = 60 W = 0.06 kW.Time used, t = 6 hEnergy = 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’.