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# Lect09 - Physics 111 Lecture 9 Todays Agenda Work Energy...

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Physics 211: Lecture 9, Pg 1 Physics 111: Lecture 9 Physics 111: Lecture 9 Today Today ’s Agenda s Agenda z Work & Energy Î Discussion Î Definition z Dot Product z Work of a constant force Î Work/kinetic energy theorem z Work of multiple constant forces z Comments

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Physics 211: Lecture 9, Pg 2 Work & Energy Work & Energy z One of the most important concepts in physics Î Alternative approach to mechanics z Many applications beyond mechanics Î Thermodynamics (movement of heat) Î Quantum mechanics. .. z Very useful tools Î You will learn new (sometimes much easier) ways to solve problems
Physics 211: Lecture 9, Pg 3 Forms of Energy Forms of Energy z Kinetic Kinetic : Energy of motion . Î A car on the highway has kinetic energy. Î We have to remove this energy to stop it. Î The breaks of a car get HOT ! Î This is an example of turning one form of energy into another (thermal energy).

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Physics 211: Lecture 9, Pg 4 Mass = Energy Mass = Energy (but not in Physics 211) (but not in Physics 211) z Particle Physics: + 5,000,000,000 V e- - 5,000,000,000 V e+ E = 10 10 eV (a) E = MC 2 (b) M ( poof ! ) (c)
Physics 211: Lecture 9, Pg 5 Energy Conservation Energy Conservation z Energy cannot be destroyed or created. Î Just changed from one form to another. z We say energy is conserved energy is conserved ! Î True for any isolated system. Î i.e. when we put on the brakes, the kinetic energy of the car is turned into heat using friction in the brakes. The total energy of the “car-brakes-road-atmosphere” system is the same. Î The energy of the car “alone” is not conserved. .. » It is reduced by the braking. z Doing “ work work ” on an isolated system will change its “ energy energy ”... Returning Can Wilberforce

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Physics 211: Lecture 9, Pg 6 Definition of Work: Definition of Work: Ingredients: Ingredients: Force ( F ), displacement ( r ) Work, W , of a constant force F acting through a displacement r is: W = F i r = F r cos θ = F r r θ F r d s p l a c e m n t F r “Dot Product”
Physics 211: Lecture 9, Pg 7 Hairdryer Definition of Work. .. Definition of Work. .. z Only the component of F along the displacement is doing work.

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Lect09 - Physics 111 Lecture 9 Todays Agenda Work Energy...

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