Lecture_10_(Student_Handoutl)

# Lecture_10_(Student_Handoutl) - PHYS 172 Modern Mechanics...

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1 Summer 2009 PHYS 172: Modern Mechanics Lecture 10 – The Energy Principle Read 5.1 – 5.6 Reading Quiz #1 & 2 Reading Question (Sections 5.1 – 5.6) (This is a closed-book quiz, no consulting with neighbors, etc.)

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2 Energy Principle Δ E = W + Q Today: The Energy Principle & Work Kinetic Energy Work What Is Energy? Energy is 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? In classical mechanics, one can derive the energy principle from the momentum principle. However, for many problems, using energy is much simpler than using the momentum principle . Often we’ll use both principles in combination.
3 The Energy Principle “cause” (due to interactions with the system from surroundings) Compare With Momentum Principle: system net pF t Δ= Δ G G “effect” (system) Energy is A Conserved Quantity 0 system surroundings EE Δ+ Δ = Compare with Conservation of Momentum: 0 system surroundings pp Δ = GG Reminder: Momentum is a vector. Energy is a scalar (no direction).

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Lecture_10_(Student_Handoutl) - PHYS 172 Modern Mechanics...

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