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Chapter 14

Chapter 14. Work & Simple Machines. Ch 14.1 – Work & Power. Work – occurs when a force causes an object to move in the direction of the applied force 1. Work involves motion, not just effort.

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Chapter 14

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  1. Chapter 14 Work & Simple Machines

  2. Ch 14.1– Work & Power Work – occurs when a force causes an object to move in the direction of the applied force 1. Work involves motion, not just effort

  3. 2. Work is done only when the force you exert on an object is in the same direction as the object’s motion a) Ex: Lifting a clothes basket is work but carrying it while walking is not

  4. 3. When a force is exerted at an angle, only the part of the force that is in the same direction as the motion is doing the work

  5. B. Calculating Work 1. Formula: Work = force x distance 2. SI Unit: Joule 3. The distance in a work equation is the distance an object moves only while the force is applied

  6. Calculating Work • A painter lifts a can of paint that weighs 40N a distance of 2m. How much work does he do? • Hint – To lift an object you must apply a force that equals the weight Work = Force x Distance Force Needed = 40 N Distance moved = 2 m 40 N x 2 m = 80 J

  7. C. Calculating Power 1. Power is how quickly work can be done 2. Formula: Power = work done / time needed 3. SI Unit: Watt

  8. Calculating Power • You do 200J of work in 12s. How much power did you use? • Power = work = 200J = 17 watts • time12s

  9. 4. Doing work on an object increases its kinetic energy 5. The amount of work done is the amount of energy transferred and therefore can also be expressed as: Power = energy transferred /time needed 6. Power is the rate at which energy is transferred

  10. Ch 14.2– Using Machines A. Machine – device that makes doing work easier

  11. B. Machines do not change the amount of work that needs to be done; they change the way a person does the work • Input force – the effort/work that you exert on the machine • Output force – the resistance force or the work the machine does

  12. 3. When using a machine the output work can never be greater than the input work a) Some make work easier by allowing a smaller force over a longer distance

  13. b) Others allow you to exert your force over a shorter distance

  14. c) Others allow you to change the direction of your force

  15. 4. Mechanical Advantage – number of times the input force is multiplied by a machine a) Formula: MA = output force / input force b) There is no SI unit for MA

  16. Calculating Mechanical Advantage • To pry the lid off a paint can, you apply a force of 50N to the handle of a screwdriver. What is the MA of the screwdriver if it applies a force of 500N to the lid? • MA = F out = 500N = 10 • F in 50N

  17. C. Efficiency – ability of a machine to convert input work to output work • Formula: • Efficiency = output work / input work x 100% • 2. Answer will be expressed as a percentage

  18. Calculating Efficiency • Using a pulley system, a crew does 7500J of work to load a box that required 4500J of work. What was the efficiency of the pulley system? • Eff = W out = 4500J x 100% = 60% • W in 7500J

  19. 3. Friction always reduces the efficiency of a machine as some of the work is converted into heat 4. The efficiency of a machine will never be 100% due to friction (Ideal Machine) 5. Oil or another lubricant can increase efficiency by reducing the number of contact points between touching surfaces

  20. Ch 14.3– Simple Machines Simple Machine – does work with only one movement Compound Machine – made of a combination of simple machines

  21. C. Types of Simple Machines Inclined Plane – a flat, sloped surface a) Allows an object to be moved from one height to another with less force than is needed to lift it b) The longer the inclined plane the less force that is needed

  22. 2. Wedge – an inclined plane that moves; may have one or two sloped sides a) Can be used to separate (cut) objects or hold an object in place b) Ex: teeth, knife, door stop

  23. 3. Screw – inclined plane wrapped around a cylinder or post

  24. 4. Lever – any rigid rod or plank that pivots around a point The fixed point about which the lever pivots is called the fulcrum There are 3 classes of levers:

  25. a) First-class lever – fulcrum is between input and output forces; ex: scissors

  26. bb) Second-class lever – output force is between the input and the fulcrum; ex: wheelbarrow

  27. cc) Third-class lever – input force is between the output and the fulcrum; ex: baseball bat

  28. 5. Wheel and Axle – made of two circular objects of different sizes that rotate together a) For some, input force turns the wheel and axle exerts the output force Ex: doorknob, steering wheel, screwdriver

  29. b) For others, the input force turns the axle and the wheel exerts the output force Ex: Ferris wheel

  30. 6. Pulley – grooved wheel with a rope or chain wrapped around it; they change the direction of the force you exert a) Fixed pulley – attached to an overhead structure; ex: flagpole

  31. b) Moveable pulley – attached to the object being lifted; must also have a fixed pulley to change direction of force

  32. c) Pulley System – combination of fixed and moveable pulleys

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