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Lecture3

# Lecture3 - In class temperature pressure units example...

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1 (- In class temperature, pressure, units, example.) Lecture 3 Work, Energy, and Heat (Prof. G. Buzyna with modifications by J.C. Ordonez) Work Work is a mechanical concept given by the expression: = 2 1 12 s d F W r r Note: F is a force and s is a displacement work is a scalar product () ds F s d F θ cos = r r , where is the angle between the force and displacement vectors only force components along the displacement vector do work force components perpendicular to the displacement vector do no work Mechanical Energy We shall consider two components of mechanical energy, kinetic and potential energy. A relationship between kinetic energy and work is obtained from Newton’s second law of motion. In scalar form dV dV ds dV Fm am m m V dt ds dt ds == = = rewriting 2 1 2 Fds mVdV md V For a displacement from s 1 to s 2 θ F d s

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2 () ()() 1 2 2 1 2 1 2 2 2 1 2 1 2 2 1 2 1 KE KE mV mV V md s d F = = = r r Work = change in kinetic energy!
3 An expression for the gravitational potential energy is obtained from the work of a weight. ( ) ( ) ()() 2 1 1 2 2 1 2 1 PE PE y y mg dy mg s d F = = = r r Note that the negative sign inside the second integral indicates the opposite direction between the displacement and the force vectors. Hence work done to lift the weight is negative. Work done on the system is considered negative and by the system positive.

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Lecture3 - In class temperature pressure units example...

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