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# lec2 - Chapter 2 Energy and the 1st Law of Thermodynamics...

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Chapter 2: Energy and the 1 st Law of Thermodynamics The Study of Energy in Closed Systems Work & Power = = 2 1 2 1 s s s d F W s d F dW r r r r • Work is the (dot) product of force and displacement F ds • Power is the rate of change of work v F dt s d F dt dW p r r r r = = = • For rotational systems ω τ = = r r dt dW p

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Energy: The Property • Common Units – J(N·m) or kJ – ft·lbf –B tu ) ( 2 1 2 1 2 2 V V m KE = ) ( 1 2 z z g m PE = • Kinetic Energy • Potential Energy Conservation of Energy dt v d m mg F ma F r r = = j • By the Newton’s 2 nd Law F ds • Multiplying (dot-product) both sides by ds and integrating yields -mg k i k ( ) 43 42 1 1 KE 2 1 2 2 PE 1 2 2 1 2 1 ) ( + = v v m z z mg W Where is the work done on the system by force F: = 2 1 2 1 s s s d F W r r
• Work can either be done on the system or by the system on the surrounding environment. • Work Sign Convention – W > 0 : Work done by the system – W < 0 : Work done on the system – Time rate of work is Power: Work Energy Transfer W Conservation of Energy: Mechanics Change in amount of energy contained within the system during some time interval + Net amount of energy

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lec2 - Chapter 2 Energy and the 1st Law of Thermodynamics...

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