Work, Power, and Machines - PowerPoint PPT Presentation

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Work, Power, and Machines

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  1. Work, Power, and Machines 9.1

  2. Work • A quantity that measures the effects of a force acting over a distance • Work = force x distance • W = Fd

  3. Work • Work is measured in: • Nm • Joules (J)

  4. Work Example • A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?

  5. Work Example • Work = Fd • Work = (5200 N)(25m) • Work = 130000 N  m = 130000 J

  6. Power • A quantity that measures the rate at which work is done • Power = work/time • P = W/t

  7. Power • Watts (W) is the SI unit for power • 1 W = 1 J/s

  8. Power Example • While rowing in a race, John uses 19.8 N to travel 200.0 meters in 60.0 s. What is his power output in Watts?

  9. Power Example • Work = Fd • Work = 19.8 N x 200.0 m= 3960 J • Power = W/t • Power = 3960 J/60.0 s • Power = 66.0 W

  10. Machines • Help us do work by redistributing the force that we put into them • They do not change the amount of work

  11. Machines • Change the direction of an input force (ex car jack)

  12. Machines • Increase an output force by changing the distance over which the force is applied (ex ramp) • Multiplying forces

  13. Mechanical Advantage • A quantity that measures how much a machine multiples force or distance.

  14. Mechanical Advantage Inputdistance Mech. Adv = Output Distance Output Force Mech. Adv. = Input Force

  15. Mech. Adv. example • Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

  16. Mech. Adv. Example • Input = 6.0 m • Output = 1.5 m • Mech. Adv.=6.0m/1.5m • Mech. Adv. = 4.0

  17. Simple Machines 9.2

  18. Simple Machines • Most basic machines • Made up of two families • Levers • Inclined planes

  19. The Lever Family • All levers have a rigid arm that turns around a point called the fulcrum.

  20. The Lever Family • Levers are divided into three classes • Classes depend on the location of the fulcrum and the input/output forces.

  21. First Class Levers • Have fulcrum in middle of arm. • The input/output forces act on opposite ends • Ex. Hammer, Pliers

  22. First Class Levers Input Force Output Force Fulcrum

  23. Second Class Levers • Fulcrum is at one end. • Input force is applied to the other end. • Ex. Wheel barrow, hinged doors, nutcracker

  24. Second Class Levers Output Force Fulcrum Input Force

  25. Third Class Levers • Multiply distance rather than force. • Ex. Human forearm

  26. Third Class Levers • The muscle contracts a short distance to move the hand a large distance

  27. Third Class Levers Output distance Input Force Fulcrum

  28. Pulleys • Act like a modified member of the first-class lever family • Used to lift objects

  29. Pulleys Output Force Input force

  30. The Inclined Plane • Incline planes multiply and redirect force by changing the distance • Ex loading ramp

  31. The Inclined Plane • Turns a small input force into a large output force by spreading the work out over a large distance

  32. A Wedge • Functions like two inclined planes back to back

  33. A Wedge • Turns a single downward force into two forces directed out to the sides • Ex. An axe , nail

  34. Or Wedge Antilles from Star Wars

  35. Not to be mistaken with a wedgIEEEEE

  36. A Screw • Inclined plane wrapped around a cylinder

  37. A Screw • Tightening a screw requires less input force over a greater distance • Ex. Jar lids

  38. Compound Machines • A machine that combines two or more simple machines • Ex. Scissors, bike gears, car jacks