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Work and Kinetic Energy

Work and Kinetic Energy. Energy (and mass) is Conserved. Energy is “Conserved” meaning it can not be created nor destroyed Can change form Can be transferred Total Energy does not change with time. This is a BIG deal!. 10. Energy. Forms Kinetic Energy Motion (Today)

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Work and Kinetic Energy

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  1. Work and Kinetic Energy

  2. Energy (and mass) is Conserved • Energy is “Conserved” meaning it can not be created nor destroyed • Can change form • Can be transferred • Total Energy does not change with time. • This is a BIG deal! 10

  3. Energy • Forms • Kinetic Energy Motion (Today) • Potential Energy Stored (Wednesday) • Heat later • Mass (E=mc2) p122 • Units Joules = kg m2 / s2 12

  4. F Dr F Dr F Dr Dr F Work by Constant Force A) W>0 B) W=0 C) W<0 • Only component of force parallel to direction of motion does work! • W = F Dr cos q 1) 2) 3) 4) 18

  5. Example: Ball Toss You toss a ball in the air. What is the work done by gravity as the ball goes up? A) Positive B) Negative C) Zero What is the work done by gravity as the ball goes down? A) Positive B) Negative C) Zero 20

  6. T N f mg Work by Constant Force • Example:You pull a 30 N chest 5 meters across the floor at a constant speed by applying a force of 50 N at an angle of 30 degrees. How much work is done by the 50 N force? 50 N 30 21

  7. Dr 90 mg T N f f mg Dr 180 Where did the energy go? • Example:You pull a 30 N chest 5 meters across the floor at a constant speed, by applying a force of 50 N at an angle of 30 degrees. • How much work did gravity do? • How much work did friction do? 25

  8. Kinetic Energy: Motion • Apply constant force along x-direction to a point particle m. • Work changes ½ m v2 • Define Kinetic Energy K = ½ m v2 W = D K For Point Particles 35

  9. N f mg y x Example: Block w/ friction • A block is sliding on a surface with an initial speed of 5 m/s. If the coefficent of kinetic friction between the block and table is 0.4, how far does the block travel before stopping? Y direction: F=ma Work W = D K 5 m/s 44

  10. Falling Ball Example • Ball falls a distance 5 meters, What is final speed? Only force/work done is gravity mg

  11. 5kg Example: block on incline A 5kg block is at rest on a frictionless incline. It is pulled 4.5m by a 25N force. What is the final velocity of the block? 25N 4.5m 30o

  12. Force F=kx Distance Work by Variable Force Force • W = FxDx • Work is area under F vs x plot • Spring F = k x • Area = ½ k x2 =Wspring Work Distance Work 48

  13. Summary • Energy is Conserved • Work = transfer of energy using force • Can be positive, negative or zero • W = F d cos(q) • Kinetic Energy (Motion) • K = ½ m v2 • Work = Change in Kinetic Energy • S W = DK 50

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