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P. Sci.

P. Sci. Unit 3 Work, Power, and Machines. Work. When a force causes an object to move – work is done. Work cont. Work = Force x distance. Or W = F x d. If the object does not move then no work is done. W = F x d. If d = 0. any number times 0 is 0 so no work.

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P. Sci.

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  1. P. Sci. Unit 3 Work, Power, and Machines

  2. Work • When a force causes an object to move – work is done.

  3. Work cont. Work = Force x distance Or W = F x d

  4. If the object does not move then no work is done. W = F x d If d = 0 any number times 0 is 0 so no work.

  5. Work also depends on direction. • The force has to be in the same direction as the motion or no work is done on the object. Lifting the Books Carrying the Books Force Force & Motion The same & Motion perpendicular Work is Not Done Work is done

  6. The SI unit for work is joules (J) F = N= kg m/s2 d = m So W = F x d = Nm 1 J = 1kg x m2/s2 = 1 Nm

  7. Power • The rate at which work is done Remember that a rate is something that occurs over time

  8. work Power = time The SI unit for Power is watts (W) Or W P = t

  9. A watt is the amount of power required to do 1 J of work in 1 s So P= W/t P= J/s Watts = J/s

  10. Machine – a device that makes doing work easier by…

  11. increasing the force that can be applied to an object. (car jack)

  12. increasing the distance over which the force can be applied. (ramp)

  13. by changing the direction of the applied force. (opening the blinds)

  14. Work In Effort force – FE(Force in) The force applied to the machine (usually by you) Work in – Win(Force in x distance in) The work done by you on the machine

  15. Work Out Resistance force – FR(Force out) The force applied by the machine to overcome resistance Work out – Wout (Force out x distance out) The work done by the machine

  16. Ideal machine Win = Wout 100% energy transfer. There is no such thing as an ideal machine – you always lose some energy (through friction, air resistance, etc.)

  17. Efficiency – a measure of how much of the work put into a machine is changed into useful output work by the machine. (less heat from friction)

  18. efficiency = (Wout / Win ) x 100% Win is always greater than Wout

  19. Mechanical Advantage • How much a machine multiplies force or distance output force (FR) • MA = input force (FE) Or input distance output distance

  20. Lever MA = Length of effort arm Length of resistance arm Remember that Length is the same as distance or LE LR

  21. Inclined Plane MA = effort distance Resistance distance or length of slope or l height of slope h

  22. Pulley • The MA of a pulley or pulley system is equal to the number of pulleys or ropes supporting the load being lifted. • A single fixed pulley only changes the direction so MA = 1

  23. Making Work Easier • Lever • Pulley • Wheel & Axle • Inclined Plane • Screw • Wedge III. The Simple Machines

  24. Effort arm You apply your force Resistance Arm Work is done here. Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes A. Lever • Lever • a bar that is free to pivot about a fixed point, or fulcrum

  25. Effort arm length Resistance arm length A. Lever • Ideal Mechanical Advantage (IMA) • frictionless machine • Le must be greater than Lr in order to multiply the force.

  26. A. Lever • First Class Lever • can increase force, distance, or neither • changes direction of force • Ex: hammer, seesaw

  27. First Class Lever

  28. A. Lever • Second Class Lever • always increases force • Ex: wheelbarrow

  29. Second Class Lever

  30. A. Lever • Third Class Levers • always increases distance • Ex: tweezers, bat, human body

  31. Third Class Lever

  32. B. Pulley • Pulley • grooved wheel with a rope or chain running along the groove • a “flexible first-class lever” F Le Lr

  33. B. Pulley • Ideal Mechanical Advantage (IMA) • equal to the number of supporting ropes IMA = 0 IMA = 1 IMA = 2

  34. B. Pulley • Fixed Pulley • IMA = 1 • does not increase force • changes direction of force

  35. B. Pulley • Movable Pulley • IMA = 2 • increases force • doesn’t change direction

  36. B. Pulley • Block & Tackle • combination of fixed & movable pulleys • increases force (IMA = 4) • may or may not change direction

  37. C. Wheel and Axle • Wheel and Axle • two wheels of different sizes that rotate together • a pair of “rotating levers” Wheel Axle

  38. effort radius resistance radius C. Wheel and Axle • Ideal Mechanical Advantage (IMA) • effort force is applied to wheel • axle moves less distance but with greater force

  39. h l D. Inclined Plane • Inclined Plane • sloping surface used to raise objects

  40. E. Screw • Screw • inclined plane wrapped in a spiral around a cylinder

  41. F. Wedge • Wedge • a moving inclined plane with 1 or 2 sloping sides

  42. F. Wedge • Zipper • 2 lower wedges push teeth together • 1 upper wedge pushes teeth apart

  43. 4. Wedges

  44. Machines IV. Using Machines Compound Machines Efficiency Power

  45. A. Compound Machines • Compound Machine • combination of 2 or more simple machines

  46. A. Compound Machines A Rube Goldberg machine, contraption, invention, device, or apparatus is a deliberately over-engineered or overdone machine that performs a very simple task in a very complex fashion, usually including a chain reaction. The expression is named after American cartoonist and inventor Rube Goldberg • Rube Goldberg Machine

  47. A. Compound Machines Rube Goldberg walks in his sleep, strolls through a cactus field in his bare feet, and screams out an idea for self-operating napkin: As you raise spoon of soup (A) to your mouth it pulls string (B), thereby jerking ladle (C) which throws cracker (D) past parrot (E). Parrot jumps after cracker and perch (F) tilts, upsetting seeds (G) into pail (H). Extra weight in pail pulls cord (I), which opens and lights automatic cigar lighter (J), setting off sky-rocket (K) which causes sickle (L) to cut string (M) and allow pendulum with attached napkin to swing back and forth thereby wiping off your chin. After the meal, substitute a harmonica for the napkin and you'll be able to entertain the guests with a little music. • Rube Goldberg Machine

  48. B. Efficiency • Efficiency • measure of how completely work input is converted to work output • always less than 100% due to friction

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