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Work/Power/Machines

Work/Power/Machines. Adapted from: Prentice Hall “Motion, Forces and Energy” By Jim Barnaby. ???Work???. What is work? List answers Which of your suggestions require energy and which do not?. Work/Energy. All work requires energy Work and energy are the same thing. Work Defined.

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Work/Power/Machines

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  1. Work/Power/Machines Adapted from: Prentice Hall “Motion, Forces and Energy” By Jim Barnaby

  2. ???Work??? What is work? List answers Which of your suggestions require energy and which do not?

  3. Work/Energy All work requires energy Work and energy are the same thing

  4. Work Defined Work:   When force moves an object Work = force  Distance W = F D = m a D   Unit: joule (j)   1 j = 1 N m

  5. Work Problems 1 joule = work done to lift a ¼ lb hamburger (1 N) 1 meter Austin lifts a 200 N box 4 meters. How much work did he do?

  6. Work Problems Chase lifts a 100 kg (220 lbs) barbell 2 meters. How much work did he do? Caitlin pushes and pushes on a loaded shopping cart for 2 hours with 100 N of force. The shopping cart does not move. How much work did Caitlin do?

  7. Power Defined Power:   Rate at which work is done Power = Work = F D = m a D Time t t

  8. Power Rangers What is the unit of power? What? What?   Unit: watt 1 watt = 1 j/s Unit: 1 j/s = 1 N m/s = 1 watt (w)

  9. Power in our life Compare cost of appliance to power. In store look at appliances (garbage disposals, blow driers, heaters, driers, leaf blowers…etc) and compare power in watts to cost.

  10. Power Humor What did the light bulb say to his mom? I wuv you watts! Who is the most powerful teacher at the Web? Ms. Watts A 100 watt light bulb does 100 j of work each second

  11. Power Problems Korey does 3000 j of work in 6 seconds. What was his power?

  12. Power Problems I shovel 10,000 kg (11 tons) of dirt and move the pile 50 meters in 6 hours. A bulldozer moves the same pile back in 30 minutes.   Who does the most work? Same   Who is the most powerful? Why?   Bulldozer--- less time

  13. Power Problems Danielle exerts 40 N of force to move an object 2 m in 4 seconds. What was her power? Charles bench presses 100 kg (220 lbs) 0.5 m for 20 reps in 20 seconds. What was the power of this impressive man of steel?

  14. Power Problems Nigel exerts 1000 N of force to move an object 600 meters in 1 minute. What was the power of this remarkable man? Solve study guide problems: 15-22, 25-33

  15. Energy Energy:   Ability to do work   Ability of something to cause change   Measured in joules (same as work) Energy = Work

  16. Potential Energy Potential Energy (PE):   Energy of position (stored energy)   (Example: rock on a cliff, battery, stretched rubber band, food) Work is done on object to gain PE)

  17. Kinetic Energy Kinetic Energy (KE):   Energy of motion   Object in motion has the ability to do work (Example: bowling ball hits pins, hammer hits nail, rubber band shoots paper)

  18. Energy Conservation Energy is conserved   Energy in = Energy out   Work in = Work out Momentum in = Momentum out

  19. Rubber Band Energy Does a rubber band have energy to do work? (un-stretched rubber bands have no energy to do work) Stretch rubber band—Does it have energy to do work? (stretched rubber bands have energy to do work)

  20. Rubber Band Energy Where is the energy when the rubber band is stretched? (it is stored in the rubber band, Elastic PE) What happens to the energy when the rubber band is released? (it is converted into motion -- PE converts into KE)

  21. Rubber Band Energy Rubber band can do work on paper wad. What happens if stretched farther?

  22. Main Idea • People have depended on machines for thousands of years to make their lives easier and more enjoyable • Most machines are complex mechanical systems that are designed to perform some overall function. • A complex mechanical system is made up of many subsystems composed of simple machines.

  23. Mechanical System Mechanical System: A machine composed of more than one simple machine

  24. Machines impact lives Q: If machines, convenience items and fast food make our lives easier (we no longer hunt or grow our own food), why do people have less time than ever before in history? See word document for machines in order (next slide has answers)

  25. Chronological order 1. Weighing scales 3500 BC 2. Gears 100 BC 3. Mechanical Clocks 1300 4. Steam locomotive 1804

  26. Chronological order 5. Electric Motor 1830 6. Internal Combustion engine 1860 7. Motorcycle 1885 8. Gasoline powered lawn mower 1902

  27. Chronological order 9. Blender 1923 10. Automatic clothes washing machine 1937 11. Jet aircraft 1939 12. CD player 1979

  28. Machines Machine:   A device that helps you do work   All machines are made up of one or more simple machines   Machines make work easier by changing the size or direction of force

  29. Machines/Energy Machines obey the law of Conservation of Energy Energy or Energy or Friction Work into a = work out of + losses Machine a machine

  30. Efficiency Efficiency: Comparison of work output to work input All machines have friction losses 100% efficient machine (no friction or heat losses) does not exist

  31. Mechanical Advantage Mechanical Advantage (MA): How many times easier a machine makes your work Number of times a machine multiplies effort force No Unit for MA

  32. Mechanical Advantage You want a machine with a MA of 10 instead of 5, as work will be 10 times easier rather than 5 times easier MA = FRResistance force FE Effort force MA = DEEffort Distance DR Resistance Distance

  33. Mechanical Advantage If we apply 20N of force to move a 60N object Solve problem on board No unit on the number for MA

  34. Forces on Machines Two forces involved with machines Effort Force: (FE) Force you put on a machine Resistance Force: (FR) Force the machine is working against (often the weight of the object)

  35. Distances/Machines Effort Distance: (DE) Distance of the effort force Resistance Distance: (DR)   Distance the object moves See word document (solve problems)

  36. Lever Law Lever law:   Work in = work out F1D1 = F2D2 An 800N man is 2 meters from the fulcrum of a teeter-totter. How far away must a 400N child sit in order to balance?

  37. Mechanical Advantage A person pushes a crowbar down 2 m with 200N of force. The crowbar raises the box 0.3 m. What is the MA?

  38. Six simple machines 6 simple machines:   Inclined Plane Wedge Screw Lever Wheel and axle Pulley

  39. Inclined Plane Every day my adorable husband lifts 3000 pounds 5 feet. He does this 2, 4 or 6 times a day, depending on the day. What is he doing? Driving his car up the driveway

  40. Inclined Plane Inclined Plane: A slanted surface used to move an object from low to high or high to low position. Examples: driveway, ramp, stairs, road

  41. Inclined Plane MA’s Demo with inclined plane, spring scale and weight FR weight of object (lift with spring scale) FE spring scale drag DE length of ramp DR height of ramp Slide object and do Force MA’s (MA = FR/FE)

  42. Inclined Plane MA’s How will MA vary with height?   Steeper inclined plane = less MA   Less steep inclined plane = large MA (easier) Example: Climb a mountain - steep sections harder to climb

  43. Inclined Plane Lab Solve Inclined Plane problems in word document Demo lab. Groups of 3 – 4 do lab and turn it in. Review lab

  44. Inclined Plane Review Quick review of inclined plane: See word document for quality graphic and formulas How will MA vary with height for an inclined plane? Steeper inclined plane: less MA, more difficult to use Less steep inclined plane: larger MA, easier to use

  45. Wedge Wedge:   An inclined plane that moves Examples: knife, axe, door stoop, wood splitting wedge….. How does sharpening a knife help it do more work?

  46. Screw Screw:   An inclined plane wrapped around a cylinder   Examples: spiral staircase, mountain road, C-clamp, bolts…   Each time a screw turns, it moves a definite distance up or down. Depends on the distance between threads Draw on chalkboard

  47. Lever Lever:   A bar that rotates on a fixed point called a fulcrum Fulcrum   The fixed point a lever rotates around MA’s can be very large, depends on leverage

  48. Lever MA Theoretically, if you have a long and rigid enough lever you could lift anything (Webber) MA = DE = Effort arm length DR Resistance arm length Use word document for all 3 classes of levers and examples

  49. Lever Demos Watch Eureka video on levers Demo lever activity Students calculate MA and list class of lever Demo lever law with meter stick and weights Wheel and axle demos

  50. Wheel and Axle Wheel and Axle   A lever that moves in a circle turning another circle Examples: screw driver, socket wrench, faucet knob, steering wheel, pencil sharpener, door knob…

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