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Unformatted text preview: Chapter 5 Work and Energy Forms of Energy Mechanical Kinetic, gravitational Thermal Microscopic mechanical Electromagnetic Nuclear Energy is conserved! Work Relates force to change in energy Scalar quantity Independent of time W = ! F ! ( ! x f " ! x i ) = F # x cos $ Units of Work and Energy SI unit = Joule 1 J = 1 N ! m = 1 kg ! m 2 /s 2 W = F ! x Work can be positive or negative Man does positive work lifting box Man does negative work lowering box Gravity does positive work when box lowers Gravity does negative work when box is raised Kinetic Energy Same units as work Remember the Eq. of motion Multiply both sides by m, 1 2 mv f 2 ! 1 2 mv i 2 = ma " x KE f ! KE i = F " x v f 2 2 ! v i 2 2 = a " x KE = 1 2 mv 2 Example 5.1 A skater of mass 60 kg has an initial velocity of 12 m/s. He slides on ice where the frictional force is 36 N. How far will the skater slide before he stops? 120 m Potential Energy If force depends on distance, For gravity (near Earths surface) ! PE = " F ! x ! PE = mgh Conservation of Energy Conservative forces: Gravity, electrical, QCD Nonconservative forces: Friction, air resistance Nonconservative forces still conserve energy! Energy just transfers to thermal energy PE f + KE f = PE i + KE i ! KE = "! PE Pendulum and Track Demos Example 5.2 A diver of mass m drops from a board 10.0 m above the water surface, as in the Figure. Find his speed 5.00 m above the water surface. Neglect air resistance. 9.9 m/s Example 5.3 A skier slides down the frictionless slope as shown. What is the skiers speed at the bottom? H=40 m start finish 28.0 m/s Example 5.4 A skier slides down the frictionless 40m high slope as shown, then reaches a flat area with k =0.2....
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This note was uploaded on 07/25/2008 for the course PHY 231C taught by Professor Pratt during the Spring '06 term at Michigan State University.
 Spring '06
 Pratt
 Physics, Energy, Force, Work

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