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General/Notes 10.1

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  • 1. ENERGY, WORK AND SIMPLE MACHINES CHAPTER 10
  • 2. ENERGY AND WORK• How do we define the relationship between work and energy?• How can we calculate work done?• How do we calculate power used?
  • 3. WORK EQUATION•
  • 4. WORK EQUATION•
  • 5. WORK EQUATION• Right side is Fd• F = force (in Newtons)• d = distance (in meters)• We define:• W = Fd• Work = forces times distance
  • 6. WORK ENERGY THEOREM•
  • 7. EXAMPLE• A 105 g hockey puck is sliding across the ice. A player exerts a constant 4.50 newton force over a distance of 0.150 m. How much work does the player do on the puck? What is the change in energy?
  • 8. LIFTING A BOOK• When is the work positive?• When is the work negative?• When is the work zero?
  • 9. WORK AGAINST GRAVITY• W = Fd• The work of lifting something is equal to the weight of the object times the distance lifted• Weight =• So W =
  • 10. WORK• Since work equals the change in KE, the unit is the same• Work is measured in joules• One joule happens when a force of 1 N acts for 1 m• An apple is approximately a newton, so lifting an apple 1 meter is about 1 Joule of work
  • 11. WORK• What if our force is not applied in a straight line?• Will it be as effective?• How do we account for this?
  • 12. WORK• W = Fdcosɵ• ɵ is between the force and the direction of displacement• If he pushes the car 10.0 m, how much work did the man do?
  • 13. WHAT TO INCLUDE IN WORK• Which direction do the normal force and gravity point?• ɵ is …• What about friction?
  • 14. EXAMPLE• A sailor pulls a boat a distance of 30.0 m along a dock using a rope that makes a 25.0° angle with the horizontal. How much work does the sailor do on the boat if he exerts a force of 255 N on the rope?
  • 15. TRIG REFRESHER• SOH-CAH-TOA
  • 16. WORK AGAINST GRAVITY• Pushing up a ramp, walking up stairs• What do we use for d?
  • 17. HOMEWORK• Page 287, # 1 – 3• Page 291, # 4 - 8
  • 18. GRAPHICAL METHOD• Area under force vs displacement curve is work• How much work?
  • 19. GRAPHICAL METHOD• Force exerted by a spring• Work =• Area of a trapezoid=• ½ h (b1 + b2)
  • 20. MULTIPLE FORCES• If several forces are exerted on a system, calculate the work done by each force, then add the results
  • 21. POWER•
  • 22. POWER• Three student going up stairs• If they started at the same time…• How does their work compare?• How does their power compare?
  • 23. POWER• On a ten-speed bike, there is a combination of force and speed that will produce the maximum power
  • 24. EXAMPLE• An electric motor lifts an elevator 9.00 m in 15.0 s by exerting an upward force of 1.20 x 104 N. What power does the motor produce in kW?
  • 25. HOMEWORK• Page 264, # 9 – 14• Page 265, # 15 - 21

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