# Lect_14 - ACT Zero net work The system of pulleys shown...

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Lecture 14 Work: Varying forces and curved trajectories Power ACT: Zero net work ACT: Zero net work The system of pulleys shown below is used to lift a bag of mass M at constant speed a distance h from the floor. What is the work done by the person? A. Mgh B. ½ C. 2 The force by the person is Mg /2 (see lecture 10), but he needs to pull on a length of rope of 2 h. net 0 (constant speed) W KE = ∆= ( ) g T WM =− =− − The story so far: net WK =∆ Kinetic energy 2 1 2 Km v = r F = = = cos // θ G G Work by a constant force, along a straight path: Work/Kinetic energy theorem The journey is divided up into a series of segments over which the force is constant. Work by non-constant force, with straight line trajectory An object moves along the x-axis from point x 1 to x 2 . A non-constant force is applied on the object. What is the work done by this force? x 1 A A B B C C D D E E E Ex D Dx C Cx B Bx A Ax + + + + = The total work is the sum of the works for each of the intervals: 2

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F x A B C D E Work=area A B C D E Ex Dx Cx Bx Ax W + + + + = This is still an approximation, because the force is not really constant in each interval… x Work=Area under ( ) curve 1 2
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## This note was uploaded on 04/16/2008 for the course PHYSICS 221 taught by Professor Johnson during the Fall '06 term at Iowa State.

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Lect_14 - ACT Zero net work The system of pulleys shown...

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