Work and Kinetic Energy

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Presentation transcript:

Work and Kinetic Energy 1

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! Maybe do example of conservation (like cookies) and counter example. Force is NOT conserved. 10

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

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 Dr F 1) 2) 3) 4) Dr F Dr F Dr F 18

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? 20

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? T mg N f 50 N 30 21

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? mg 90 Dr T mg N f f Dr 180 25

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

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? x y mg N f Y direction: F=ma Work W = D K On transparency 5 m/s 44

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

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 5kg 30o

Work by Variable Force W = Fx Dx Work is area under F vs x plot Work Spring F = k x Area = ½ k x2 =Wspring Force Work Distance Force Distance F=kx Work 48

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