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Chapter 5

Chapter 5. Work and Machines. Work. The transfer of energy to cause or make an object move. Work. If there is no movement, no work is done Ex: Lifting a text book (work) vs pushing on a brick wall (no movement = no work) . Work and energy.

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Chapter 5

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  1. Chapter 5 Work and Machines

  2. Work • The transfer of energy to cause or make an object move

  3. Work • If there is no movement, no work is done • Ex: Lifting a text book (work) vs pushing on a brick wall (no movement = no work)

  4. Work and energy • When work is done, a transfer of energy occurs • You become tired when you walk or carry things up a flight of stairs

  5. Joule • The unit used to express work (J)

  6. How do you calculate work? • Work = Force X Distance • Or • W = F x D

  7. Try It • You apply a force of 10 N to a shopping cart. You moved the cart 10 m. How much work did you do on the shopping cart?

  8. Power • The rate at which work is done or energy is transferred

  9. watt • The unit used to express power. • Symbol for Watt (W) • Usually written in italics

  10. How do you calculate Power? • Power = Work/Time • Or • P = W/T

  11. Try It • It takes you 5 s to do 100 J of work on a shopping cart to move it down the sidewalk. What is your power output?

  12. machines • A device that makes doing work easier • Changes force (increases it) • DOES NOT change the amount of work

  13. Work done by machines • Input force – force that is applied to the machine = Fin • Output force – the force applied by the machine = Fout

  14. Work input • B/c of friction machines aren‘t100% efficient • Work Input= force YOU exert on machine = Win • Input Distance = distance YOU are using • Ex: Rowing boat • Input force - how hard you pull oars • Input distance - how much oar handles move

  15. Work output • Output Force = force exerted by machine • Output distance = distance machine uses • Work Output = what machine does/accomplishes = Wout • Ex: Rowing boat Output force - how hard oars push water Output distance - distance oar ends move in water Work Output - moving through water

  16. Work input and output

  17. Conserving energy • When energy is used by a machine, some of the energy is transferred as heat due to friction • Wout is never greater than Win • Wout is always smaller than Win

  18. Mechanical advantage • Advantage of using a particular machine • Mechanical Advantage Equation: • Mechanical Advantage = output force (in newtons) • input force (in newtons) • OR • MA = Fout / Fin

  19. Try It • Calculate the mechanical advantage of a hammer if the input force is 125N and the output force is 2000N.

  20. Mechanical Efficiency • Measure of how much of the work put into a machine is changed into useful work output by the machine • Calculating Efficiency: • Efficiency(%) = output work (in joules) x 100% • input work (in joules • OR • efficiency = Wout / Win x 100% • Higher the number, the more efficient

  21. Try it • Find the efficiency of a machine that does 800J of work if the input work is 2400J.

  22. Types of Machines (Simple Machines) • 6 Simple Machines make other machines • Lever Family • Lever • Pulley • Wheel & axle • Inclined Plane Family • Simple inclined plane • Wedge • Screw

  23. Levers • Have a rigid arm that turns around a fulcrum • 3 classes: • 1st class • 2nd class • 3rd class

  24. 1st Class Lever • Fulcrum in middle of arm • Exs: scissors, pliers, hammer claw, seesaw

  25. 2nd Class Lever • Fulcrum at one end and force at other end • Load in middle • Exs: wheelbarrow, door, nutcracker • (MA > 1)

  26. 3rd Class Lever • Fulcrum at one end, force in middle, and load on other end • Exs: tweezers, biceps

  27. Pulley • Fulcrum in middle of circle • Lever = rope • More pulleys – easier work

  28. Types: A.) Fixed • wheel attached in a fixed position • MA = 1

  29. B.) Movable attached to the object being moved MA = 2

  30. C.) Block and Tackle Combination of fixed and movable pulleys MA = depends on the number of rope segments

  31. Wheel & Axle • 2 different sized wheels • Axle is fulcrum, wheel is lever • Exs: steering wheel, screw driver • Gears – toothed W & A

  32. Mech. Adv. Of a Wheel and Axle • MA = Radius of Wheel/Radius of Axle

  33. Inclined Plane • Spreads work over long distances • Easier to use a long ramp • Exs: stairs, ramps, escalators

  34. Mech. Adv. Of Incline Planes • MA = Length/Height (L/H)

  35. Wedge • 2 inclined planes back to back • Holds together or separates objects • Exs: nails, axes

  36. Wedges

  37. Screw • Threads are spiraled incline plane • Exs: jar lid, spiral staircase

  38. Screw

  39. Compound Machine • More than 1 simple machine together • Ex: scissors - lever (handles) and wedge (blade)

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