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Chapter 6 Work and Energy

Chapter 6 Work and Energy. 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. work ( N ·m or Joule ). force (N). displacement (m).

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Chapter 6 Work and Energy

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  1. Chapter 6Work and Energy

  2. 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

  3. Less work is done on the object in bottom figure.

  4. work (N·m or Joule) force (N) displacement (m)

  5. θ = 90°; cosθ = 0W = 0 θ = 0°; cosθ =1W = F(s) θ = 180°; cosθ = -1W = - F(s) Block is moving this way  θ = 270°; cosθ = 0W = 0

  6. Person is doing positive work on the barbell when lifting. Person is doing negative work on the barbell when lowering

  7. Work can be positive or negative, but it is NOT a vector. Work is measured in Joules (Newton-meters) or ft-lbs

  8. 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

  9. 6.2 – Work-Energy Theorem & KE Energy - The ability to do work; measured in Joules Kinetic Energy - Energy due to motion velocity(m/s) mass (kg)

  10. 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

  11. ASSIGNMENT Ch. 6READ 6.1 – 6.2Conceptual Questions #1-9 (p. 179)Problems #2,8,12,17

  12. 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 )

  13. 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

  14. Potential Energy - Energy due to relative position Elastic Potential Energy Electrical Potential Energy Gravitational Potential Energy

  15. 6. 3 - Gravitational Potential Energy Work done by the force of gravity height difference (m)

  16. Gravitational Potential Energy height (m) The work done by gravity does not depend on the path taken, only the height difference.

  17. 6. 3 – Conservation of Mechanical Energy The total mechanical energy (E) of an object remains constant, neglecting frictional forces. E = KE + PE Einitial = Efinal

  18. the rate at which work is done. Power - 1 horsepower = 550 ft-lbs = 745.7 watts

  19. ASSIGNMENT Ch. 6Problems #26-36 (evens) & #59 page 182 due Wednesday

  20. Ch. 6 Equations E = KE + PE Einitial = Efinal

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