Lecture_9

# Lecture_9 - REVIEW of Last Weeks Lecture B B cos Scalar Product A A B AB cos Ax Bx Ay B y Az Bz Work W Fs constant force parallel to displacement W

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P123 Lecture 9 29 October 2010 Power and Gravitational Potential Energy REVIEW of Last Week’s Lecture Scalar Product A B cos B Work z z y y x x B A B A B A AB B A cos constant force parallel to displacement F W constant not parallel to variable force along x (area under F-x curve) F s 2 2 1 dx F W s F W Fs x x x Work-Energy Theorem 2 in general 1 s d F W Kinetic Energy 1 2 1 K K K s d F W net tot 2 2 1 ; Where MV K F F r i net Spring, Hooke’s Law kx F x 1 displacement from equilibrium 2 2 1 kx W

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i Clicker cow A piece of fruit falls straight down. As it falls, the gravitational force: F s cow s F W A. Does positive work B. Does negative work C. Does no work D. First does negative work, then positive work E. First does positive work, then negative work
POWER Rate at which work is done t W P AV (Rate at which energy changes) dt dW t W P t 0 lim Units: 1 Watt = 1 Joule/sec 1 Joule = 1 WS (Light bulb ~ 100W) (Numans ~ 100W) (1 hp = 746W) EXAMPLE: A 5 kg mass falls 3.2 meters. What is the average power generated by gravity? 2 s 0.8 s 64 . 0 ) m/s 10 ( ) m 2 . 3 ( 2 2 2 2 2 1 g s t gt s J 160 ) m 2 . 3 )( m/s 10 )( kg 5 ( s mg s F W W 200 ) s 8 . 0 ( ) J 160 ( AV P The can stops the mass in 1/10 the time it takes to fall.

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## This note was uploaded on 03/14/2011 for the course P 123 taught by Professor Stevens during the Spring '11 term at Rust.

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Lecture_9 - REVIEW of Last Weeks Lecture B B cos Scalar Product A A B AB cos Ax Bx Ay B y Az Bz Work W Fs constant force parallel to displacement W

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