PHY183-Lecture13 - 1 Physics for Scientists &...

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Unformatted text preview: 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 February 9, 2011 Physics for Scientists&Engineers 1 1 Spring Semester 2011 Lecture 13 Work-Kinetic Energy, Work by Variable Force 2 s I am holding a box that weighs 25 N. I walk a distance of 4.0 m holding the box at constant speed. How much work have I done? A) 100 J B) -100 J Clicker Quiz Clicker Quiz Clicker Quiz Clicker Quiz February 9, 2011 Physics for Scientists&Engineers 1 2 C) 0 J 3 Clicker Quiz Solution Clicker Quiz Solution Clicker Quiz Solution Clicker Quiz Solution s I am holding a box that weighs 25 N. I walk a distance of 4.0 m holding the box at constant speed. How much work have I done? A) 100 J B) -100 J Although you feel tired after carrying the box, you have not done any work in the February 9, 2011 Physics for Scientists&Engineers 1 3 C) 0 J physics definition of work 4 Work Work-Kinetic Energy Theorem Kinetic Energy Theorem Work Work-Kinetic Energy Theorem Kinetic Energy Theorem s Relationship between kinetic energy of an object and the work done on the object by a constant force s This is the work-kinetic energy theorem K K K W - = February 9, 2011 Physics for Scientists&Engineers 1 4 s The work-kinetic theorem is equivalent to Newtons Second Law 1 2 mv x 2- 1 2 mv x 2 = ma x x- x ( 29 = F x x = W 5 Work Work-Kinetic Energy Theorem Kinetic Energy Theorem Work Work-Kinetic Energy Theorem Kinetic Energy Theorem s Equivalent formulations (solving for K or K ): s Kinetic energy cannot be less than zero he ork that can be done by an object (negative work) K = K + W K = K- W February 9, 2011 Physics for Scientists&Engineers 1 5 s The work that can be done by an object (negative work) cannot have a bigger magnitude than the initial kinetic energy s The work-kinetic energy theorem is valid for constant forces and variable forces s The work-kinetic energy theorem is NOT valid for friction 6 Work Work Done Done by Gravity by Gravity Work Work Done Done by Gravity by Gravity s Work done by gravitational force during upward motion of a projectile W g = -mgh (reduces kinetic energy on the way up) February 9, 2011 Physics for Scientists&Engineers 1 6 s Work done by gravitational force on the projectile on the way down W g = mgh (increases kinetic energy on the way down) 7 Work Work Done Done Lifting an Object Lifting an Object Work Work Done...
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PHY183-Lecture13 - 1 Physics for Scientists &...

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