Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 6 Work and Energy 6.1 – Work Work Formula & Units Positive & Negative Work 6.2 – Work-Energy Theorem & Kinetic Energy KE Formula & Units 6.3 – Gravitational Potential Energy GPE Formula Positive & Negative Work 6.4 – Conservation of Energy Total Mechanical Energy 6.5 – Power Power Formula 6.1 – Work Done by a Constant Force Work is done on an object whenever a force is applied parallel to the displacement. Work = Force x Displacement Less work is done on the object in bottom figure. W ( F cos ) s work (N·m or Joule) force (N) displacement (m) W ( F cos ) s θ = 90°; cosθ = 0 W=0 θ = 180°; cosθ = -1 W = - F(s) θ = 0°; cosθ =1 W = F(s) Block is moving this way θ = 270°; cosθ = 0 W=0 Person is doing positive work on the barbell when lifting. Person is doing negative work on the barbell when lowering Work can be positive or negative, but it is NOT a vector. Work is measured in Joules (Newtonmeters) or ft-lbs Are you doing work on the object? 1. Lifting a weight up off the floor. YES 2. Pushing a truck as hard as you can but the truck doesn’t move NO 3. Carrying books across a room. NO 4. Lowering a barbell during a bench-press rep. YES, negative work 5. Gravity pulling a ball down to earth. YES 6. Gravity pulling on a book resting a table. NO For now, a good way to know if work is done is to see if the PE or KE of the object is changed. Work will cause a change in energy of the object. Ch. 6 Homework #1 Ch. 6 Problems #1-5 (p. 180) 6.2 – Work-Energy Theorem & KE Energy - The ability to do work; measured in Joules Kinetic Energy - Energy due to motion 1 2 KE mv 2 velocity(m/s) mass (kg) F ma v v 2 ad Fd W mad W m v 2f v02 2 W mv mv 1 2 2 f 1 2 W KE f KEi 2 0 2 f 2 0 v 2f v 02 2 ad The Work-Energy Theorem A net external force on an object changes the KE of the object. The change in KE of the object equals the work that was done on the object W = ΔKE W KE f KEi Ch. 6 Homework #2 Ch. 6 Problems #12,13,15,17 p. 181 Potential Energy Energy due to relative position Elastic Potential Energy Electrical Potential Energy Gravitational Potential Energy 6. 3 - Gravitational Potential Energy Work done by the force of gravity Wgrav mgh height difference (m) W ( F cos ) s Wgrav (mg cos 0 )(h0 hf ) Gravitational Potential Energy PE mgh height (m) The work done by gravity does not depend on the path taken, only the height difference. 6. 4 – Conservation of Mechanical Energy The total mechanical energy (E) of an object remains constant, neglecting frictional forces. E = KE + PE Einitial = Efinal The Kingda Ka is a giant roller coaster with a vertical drop of 127 m. Suppose that the coaster has a speed of 6.0 m/s at the top of the drop. Neglect friction and air resistance and find the speed of the riders at the bottom in miles/hour Chapter 6 Homework #3 Ch. 6 Problems #25,26,28,35,32,36 page 182 6. 5 – Power Power - the rate at which work is done. Work (joules) Average Power (watts) = time (sec) 1 horsepower = 550 ft-lbs/sec = 745.7 watts Conservation of Energy Lab When block is moving up or down at constant velocity, the net force is zero. Fup = Fgrav + fk Fdown = Fgrav - fk Fup + Fdown = 2 (Fgrav ) Conservation of Energy Lab 1. W = mg 2. Fgrav = (Fup + Fdown) /2 3. Fgrav = Wsinθ 4. Work = Fgrav x length 5. ΔPE = mgh 6. Workactual = Fup x length Ch. 6 Equations W ( F cos ) s 1 2 KE mv 2 W KE f KEi Wgrav mgh Work (joules) Average Power (watts) = time (sec) E = KE + PE Einitial = Efinal