Work and Machines

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Work and Machines

• How to get the job done easier

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Work Power

• Work is the transfer of energy when a force makes
  an object move
• Work to be done
• 1. Object has to move
• 2. Direction of movement is in the direction of
  the force applied
• Work force x distance
• W F x d

• Power is the amount of work done in a certain
  amount of time
• Power work / time
• P W/t
• Measured in watts

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Energy

• Energy is transferred from the object doing the
  work to the object having the work done
• Power is used or produced anytime energy is being
  transferred

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Think?

• Which ones are work being done?
• Person shovels snow off a sidewalk
• Worker lifts bricks, one at a time, from the
  ground to the back of his truck
• Carrying a bundle of shingles across a
  construction site

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Machines make work easier

• Machine - a device that makes doing work easier
• Increases force, distance or changes force
  direction
• Two forces involved for a machine to do work
• Effort force (Fe) - force applied to the machine
• Resistance force (Fr) - force applied by the
  machine to overcome resistance

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Non-Ideal and Ideal Machine

• Non-Ideal Machine
• Some work put into the machine is lost
• Lost to heat because of friction
• Work input is always less than work output
• Win lt Wout

• Ideal Machine
• No work done to machine is lost
• No input lost because there is no friction (no
  heat produced)
• Work input equals work output
• Win Wout

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Mechanical Advantage and Efficiency

• Mechanical Advantage (MA) - number of times a
  machine multiplies the effort force
• How useful is a machine

• Efficiency - the measure of how much of the work
  put into the machine is useful output work
• Efficiency (Wout/Win) x 100
• Increase efficiency by decreasing friction

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Think?

• Why is Wout always less than Win?
• How is efficiency calculated?
• What is there not any of in an ideal machine?

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Simple Machines

• Simple Machine - machine that does work with only
  one movement
• Six Types
• Levers
• Pulleys
• Wheel and Axle
• Inclined Plane
• Screw
• Wedge

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Levers

• A bar that is free to pivot, or turn, about a
  fixed point
• Fulcrum - the fixed point on a lever
• Effort arm - distance from the fulcrum to where
  the effort force is applied

• Resistance arm - distance from the fulcrum to
  where the resistance force is applied
• Three classes of levers

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Three Classes of Levers

• Class 1 - fulcrum is between the effort and
  resistance forces
• Screwdriver opening a paint can
• Class 2 - resistance force is located between the
  effort force and fulcrum
• Wheelbarrow
• Class 3 - effort force is between the resistance
  force and the fulcrum
• Baseball bat

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Mechanical Advantage

• IMA Length of effort arm/length of resistance
  arm
• To increase mechanical advantage increase the
  effort arm

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Pulleys

• A grooved wheel with a rope, chain, or cable
  running along grooved wheel
• Pulleys multiply the effort force and change the
  direction of the effort force

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Types of Pulleys

• Fixed pulley - attached to fixed object and
  cannot move
• Only changes the direction of the effort force
• IMA 1

• Movable Pulley - one end of the rope is stable
  and the other end is free to move
• Multiplies force
• IMA 2

• Block and Tackle - System of fixed and movable
  pulleys
• IMA is equal to the number of rope sections

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Wheel and Axle

• Consists of two wheels of different sizes that
  rotate together
• Examples
• Gears
• Door knob
• Screwdriver and screw

• Mechanical Advantage
• IMA radius of wheel/radius of axle
• To increase mechanical advantage increase radius
  of wheel

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Wheel and Axle
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Inclined Plane

• A sloping surface that reduces the amount of
  force required to do work
• Examples
• Loading ramp
• Switch backs on a mountain
• Door Stop
• IMA Length of slope/height of slope
• To increase mechanical advantage increase length
  of slope making ramp less steep

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Screw Wedge

• Screw - an inclined plane wrapped in a spiral
  around a cylindrical post
• Examples
• Wood Screw
• Jar Lids
• Traveling around a mountain

• Wedge - one or two sloping side inclined plane
• Examples
• Knife
• Ax
• Log Splitter

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Think?

• Which of the six simple machines are variations
  of lever and inclined plane?
• What is the fulcrum and effort arm of bottle
  opener?
• Why would friction be good on an inclined plane?