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Lesson #1 What is energy? Potential Energy • Gravitational PE = mgh Kinetic Energy KE = ½ m 2 v Example • When you stand at the top of the football stadium you are 12m above the ground. How much gravitational potential energy do you have? Example • A 15g bullet is traveling at 450m/s. How much kinetic energy does it have? Total energy before = Total energy after (Law of conservation of energy) Energy Conversion Lab – Energy of a Tossed Ball Lesson #2 Tell me what you know… • PE • KE • Lab from last week Homework Questions What is… • Work • W = Fd • Units? No displacement, no work! For work... – Only portion of force that is parallel to displacement is considered Is work being done? Practice Problem • John pulls a sled over ice with a force of 2350 N at an angle of 25. The sled travels 25 meters before being stopped to load wood. How much work does John do? What is… • Power • W/t • Rate at which work is done • Units? What is horsepower? FYI…Equivalent to a horse walking a 24 foot diameter circle, pulling with 180 pounds of force, 144 trips per hour Conversion • 1 horsepower (hp) = 746W Practice Problem, Part 2 Vector or Scalar? • Energy • Work Work is negative when... • Force acts in the direction opposite motion Practice... • Suzy attaches a rope to a box at an angle of 33º from the horizontal. She pulls on the rope with 350 N of force over 15 m. There is a frictional force of 55 N. • How much work does Suzy do on the box? • How much work does friction do on the box? • What is the net work on the box? Work-Energy Theorem • W=KE • When work is done on an object, a change in kinetic energy results Practice... • In 1990 Roger Hickey of California reached a speed of 35.0 m/s on his skateboard. Suppose it took 21 kJ of work for Roger to reach this speed from a speed of 25.0 m/s. Calculate Hickey’s mass. Power Lab Energy problems – set 2