Lect10

# Lect10 - PHYS 172 Modern Mechanics Lecture 10 The Energy...

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Fall 2011 PHYS 172: Modern Mechanics Lecture 10: The Energy Principle Read 6.1 – 6.7 1

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TODAY • The Energy Principle • Energy is Conserved • Rest Energy and Kinetic Energy • Work = ! F ! " ! r 2
What Is Energy? Energy is just some number you can compute that obeys a conservation law. It ` s helpful book-keeping. (See Feynman Lectures on Physics, Vol. 1, Sec. 4-1) Why Introduce Energy? • say something about initial and final states of a system • predict whether some process CAN occur Without following the details of a complicated process, we can: Basically, for some problems, using energy is much simpler than using the momentum principle . Often, however, we ` ll use both principles in combination. 3

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The Energy Principle system surr E W Q ! = + l effect z l cause z (due to interactions) Compare With Momentum Principle: impulse syst m net e F p t = ! ! = ! ! l effect z l cause z (due to interactions) NOTE: Momentum is a vector. Energy is a scalar (no direction). * We mean the Work done ON a System by a Force in the Surroundings* 4
Energy is A Conserved Quantity 0 system surroundings E E ! + ! = Compare with Conservation of Momentum: 0 system surroundings p p ! + ! = ! !

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Lect10 - PHYS 172 Modern Mechanics Lecture 10 The Energy...

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