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Machines

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Machines may:

  • Change the direction of a force
  • Change the amount of force
    • This does not mean changing the amount of work or energy!!!
      • Work input = work output + thermal E
      • (f•d)input = (f•d)output + thermal E

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  • MA = Output force = input distance

Input force output distance

  • Problem 1: If you use a machine to move something requiring 50N of force on the object, and the MA is 2, how much force did you exert?

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  • If the object was displaced one meter, how far was your end of the machine displaced?

Mechanical Advantage

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Levers

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    • May have MA greater than, equal to, or less than one, and sometimes change direction of force.
      • Input distance / output distance proportional to distance between force and fulcrum (pivot point)
      • Input arm length = MA

output arm length

1st class, 2nd class, 3rd class levers: depends on

location of fulcrum, input, and output force

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Problems

  • #2: What type of lever’s output force is always greater than the input force?

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  • #3: What type of lever’s input force is always greater than output force?

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Efficiency

  • % efficiency = Work out

Work in

Can % efficiency ever equal 100?

Why / why not?

What would happen if a machine was over 100% efficient?

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Percent efficiency

  • In a perfect world, the work into a machine is equal to the work you get out.
  • Real world – we have friction

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  • >100% efficiency impossible

(Unless the goal is making thermal energy,

100% is possible there.)

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Pulleys

  • Fixed Pulley
  • One string supports load
  • Changes direction of force
  • Does not change magnitude of force
  • MA = ?

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Moveable Pulley

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  • Now two strings support load
  • MA:

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Block and Tackle

  • Series of pulleys work together
  • More than two strings support load
  • MA = # of strings supporting load
  • Demonstration: drawn together

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Wheel and Axle, or Gears

  • The larger the wheel compared to the axle, the greater the MA
  • If wheel has 5x greater circumference, MA = ?

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Inclined Planes

  • MA = Length of ramp / height of ramp
    • What is the MA of a wheelchair ramp that is 4m long and 0.5m high?
  • 4m/0.5m = 8

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  • How many times easier is this than lifting objects up 0.5m without the ramp?

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Wedge and Screw

  • Both types of modified inclined planes
  • Wedge is two inclined planes, back to back
  • Screw is inclined plane wrapped around a cylinder

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Power

  • How fast energy can be converted/transferred
  • (how fast Work can be done)
  • Measured in Watts

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Watt = Joule per second ( J )

s

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(interesting but not on exam): 1 hp = 746W

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  • Work Practice

Sarah pulls a wagon with a force of 20N for 10m.

  • Work done:

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  • #2: A bulldozer does 5000J of work on a load of dirt. If the dirt was moved 0.5m, find the force exerted on the dirt.

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  • Work practice (continued)
  • Jake weighs 500N. He climbs a flight of stairs. Afterward he has done 1000J of work. Find the height of the stairs.

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  • Ibro increases his GPE by 1800J when he climbs 3m. Find his weight.

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Power practice

  • If Ibro did his 1800J of work in 2 seconds, find his power.

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  • What about if it had taken him 4 seconds?

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Work practice (continued)

You lift a 20kg box to a height of 4m. This takes two seconds.

  • 1. Work done:
  • 2. Your power:
  • 3. If, instead of lifting the box straight up, you push this box (with wheels) up a ramp that is 16m long, what force do you have to exert?

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Power practice (cont)

  • A 60W light bulb uses how much energy per second?

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  • How about a 15W light bulb?

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  • If a 2000W motor lifts a weight and does 10,000J of work, how long did it take?

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% eff practice

  • 200J of work are done on the input side of a machine. 150J come out.

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  • This machine’s efficiency is:

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  • Where did the other energy go?
  • How much?