Lecture214Week5

# Lecture214Week5 - This Week Work Energy Power Energy makes...

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This Week Work, Energy, Power Energy makes our everyday world work Where does energy go? Are we using it up? How can one store energy? Where does energy come from. The heat of the earth Escape velocity 12/09/11 Physics 214 Fall 2011 1

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12/09/11 2 Physics 214 Fall 2011 Work, Energy and Power We all use the words Work, Energy and Power and indeed our usage is generally correct. Once again, however, we need to write down simple definitions and be able to do calculations. Energy comes in a wide variety of forms. For example if you go on a trip in your car energy is being supplied by the gasoline. Initially some of the energy is used to give the car speed but when you stop gasoline has been used but the car now has no energy. The energy went into the air you passed through, dissipated heat in the tires, brakes and engine and so on.
12/09/11 3 Physics 214 Fall 2011 Energy Conservation If we take a closed system, that is one that nothing can enter or leave, then there is a physical law that energy is conserved. We will define various forms of energy and if we examine the system as a function of time energy may change into different forms but the total is constant. Energy does not have direction just a magnitude and units. Conservation of Energy follows directly from the statement that physical laws do not change as a function of time.

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12/09/11 4 Physics 214 Fall 2011 Forms of mechanical energy One obvious form of energy is the energy of a moving object this is kinetic energy = 1/2mv 2 A second form of energy is what is called Potential Energy. This energy is the energy stored in a compressed spring or stretched elastic or in an object that is held at rest above the earths surface. When the spring or elastic or the object is released one gets kinetic energy appearing from the stored energy. In the case of a pendulum there is a continual storage and release of energy as the pendulum swings.
12/09/11 5 Physics 214 Fall 2011 Work and energy If an object initially at rest is acted on by a net force F it will accelerate and after time t will have moved a distance d We define Work W = Fd units are joules Both F and d can be + of – so W can be positive or negative Now take our usual equations v = v 0 + at d = v 0 t +1/2at 2 and F = ma Fd = ma(1/2at 2 ) = ma(1/2av 2 /a 2 ) = 1/2mv 2 kinetic energy F is the net force in the direction of motion F d +

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12/09/11 6 Physics 214 Fall 2011 Negative Work If F is in the opposite direction to the motion then Fd is negative. Remember F and d have magnitude and direction and can be positive or negative. If the work is negative energy is being removed from the object Friction always opposes motion and the work F f does is negative F d F f F W = Fd - F f d
12/09/11 7 Physics 214 Fall 2011 Net force and Work If there is more than one force acting we have to find the work done by each force and the work done by the net force Net force F – F f work = (F – F f )d = 1/2mv 2 The work the force F does is Fd and if we write the equation as Fd = F f d + 1/2mv 2 we can see that some work goes into heat and some into kinetic energy and we can account for all the work and energy F d F f

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