chapter_7

# chapter_7 - Chapter 7 Work Energy of a System Reading...

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The concept of energy (and the conservation of energy – chapter 8) is one of the most important topics in physics. Work Kinetic energy Energy approach (“system model”) is often simpler than Newton’s second law (“particle model”). Scalar product (dot product); work is scalar product of force and displacement. Hooke’s law, springs Power Chapter 7: Work; Energy of a System Reading assignment: Chapter 7 Homework : CQ1, CQ5, QQ1, QQ2, AE2, 1, 9, 11, 12, 14, 15, 17, 25, 27, 31, 33, 42, 43 Due dates: Tu/Th, and MWF section : Monday, Feb. 28 Remember HW 6 due Monday, Feb. 21

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Work (as defined by a physicist) Definition: The work done on an object by an external force is: the product of the component of the force in the direction of the displacement and the magnitude of the displacement. θ cos = d F W
How much work is done when just holding up an object? = = W d F W θ cos

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How much work is done when the displacement is perpendicular to the force? = = W d F W θ cos
What is the work done when lifting? (at constant speed) - By the applied force? - By the gravitational force? = = W d F W θ cos Sign convention: W positive: If F and d are parallel . If energy is transferred into the system. W negative: If F and d are antiparallel . If energy is transferred out of the system. How much work is the strongest man doing when he lifts a 100 kg boulder by 1 m.

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