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**Unformatted text preview: **Chapter 8: Potential Energy and Conservation of Energy
81 What Is Physics? 82 Work and Potential Energy? 83 Path Independence of Conservative Forces 84 Determining Potential Energy Values 85 Conservation of Mechanical Energy 86 Reading a Potential Energy Curve 87 Work Done on a System by and External Force 88 Conservation of Energy Chapter 8 Homework 7, 15, 16, 21, 27, 34, 38, 42, 51, 52, 53, 58 Potential Energy Potential energy is associated with a system. The value of potential energy depends on the choice of the reference point (zero potential). The change in potential energy does not depend on the choice of the reference point. Sample Problem 8-2: Potential Energy Curve
dU ( x) = - dW = - F ( x)dx dU ( x) F ( x) = - dx Turning Points
K = 0, Emec = U Equilibrium Points
F ( x) = 0, dU ( x) =0 dx Stable Unstable Neutral d 2U ( x) >0 dx 2 d 2U ( x) <0 dx 2 d 2U ( x) =0 2 dx Work Done by External Forces
W = K
W = Emec = K + U W = E mec + E th
Increase in thermal energy by sliding E th = -W f = f k d
Sample Problem 8-5: Moving object on horizontal surface with friction Sample Problem 8-6: Decelerating crate m = 14 kg, F = 40 N, d = 0.50 m, vi = 0.60 m/s, v f = 0.20 m/s Find WF , E th ? The Conservation of Energy
Energy can neither be created not destroyed, but can only be converted from one form to another. The total energy of the universe is constant. Energy can be converted from on form to another, or transmitted from one region to another, but energy can never be created or destroyed. The total energy E of a system can change only by amounts of energy that are transferred to or from the system The total energy E of an isolated system cannot change W = E mec + E th + Eint E mec + E th + Eint = 0 (isolated system)
Power
Pavg E = t dE P= dt ...

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