Pearson Prentice Hall Physical Science: Concepts in Action

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Pearson Prentice Hall Physical Science Concepts
in Action

• Chapter 14
• Work, Power and Machines

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14.1 Work and Power

• Objectives
• 1. Describe the conditions that must exist for a
  force to do work on an object
• 2. Calculate the work done on an object
• 3. Describe and calculate power
• 4. Compare units of watts and horsepower as they
  relate to power

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Conditions for Work

• Def work is the product of force times distance
• For a force to do work on an object, some of the
  force must act in the same direction as the
  object moves
• If the object does not move, no work is done
• Work depends on direction
• Any part of a force that does not act in the
  direction of motion does no work on the object

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Calculating Work

• Work Force x Distance
• The units for force are Newtons, N
• Recall from chapter 12 that 1 N 1 kgm/s2
• The unit for distance is the meter, m
• The unit for force is 1 Nm or 1 kgm2/s2 which
  equals one joule, abbreviated J

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Calculate Power

• Def power is the rate of doing work
• Doing work at a faster rate requires more power
• To increase power, increase the amount of work
  done in a given time OR do a given amount of work
  in less time
• Power Work/Time
• The unit of work is joules (J)
• The unit of time is seconds (s)
• J/s watts (W) the unit of power is watts

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Watts and Horsepower

• One horsepower equals 746 watts
• James Watt defined horsepower as the power output
  of a very strong horse
• Watt did not want to exaggerate the power of
  steam engines

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14.2 Work and Machines

• Objectives
• 1. Describe what a machine is and how it makes
  work easier to do
• 2. Relate work input of a machine to work output
  of the machine

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What a Machine is How it Makes Work Easier

• Def a machine is a device that changes a force
• Machines make work easier to do
• Machines change the size of a force needed, the
  direction of the force, or the distance over
  which a force acts
• Some machines increase distance over which to
  exert a force, decreasing the amount of force
  needed
• Some machines exert a large force over a short
  distance
• Some machines change the direction of the applied
  force

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Work Input and Work Output

• Because of friction, the work done BY a machine
  is always less than the work done ON a machine
• Def work input is work done by the input force
  acting through input distance
• Def work output is force exerted by a machine
• Def output distance is the distance of the
  output force

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14.3 Mechanical Advantage and Energy

• Objectives
• 1. Compare a machines actual mechanical
  advantage to it ideal mechanical advantage
• 2. Calculate the ideal and actual mechanical
  advantages of various machines
• 3. Explain why efficiency of a machine is always
  less than 100
• 4. Calculate a machines efficiency

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Actual and Ideal Mechanical Advantage
Calculations

• Def mechanical advantage is the number of times
  that a machine increases an input force
• Actual MA output force/input force
• Def ideal mechanical advantage is the MA in the
  absence of friction
• Friction is always present, so the actual MA of a
  machine is always less than the ideal MA
• Ideal MA input distance/output distance
• There are no units with MA

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Efficiency Calculation Why it is Less Than 100

• Def efficiency of a machine is the percentage of
  work input that becomes work output
• Efficiency is always less than 100 since
  friction is always present
• Efficiency work output/work input x 100

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14.4 Simple Machines

• Objectives
• 1. Describe the six types of simple machines
• 2. Explain what determines the mechanical
  advantage of the six types of simple machines

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Six Types of Simple Machines MA

• The six types of simple machines are the lever,
  wheel and axle, inclined plane, wedge, screw and
  pulley
• Def a lever is a rigid bar free to move about a
  fixed point
• Def a fulcrum is the fixed point a lever moves
  around
• Def the input arm is the distance between the
  input force and the fulcrum

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• Def the output arm is the distance between the
  output force and the fulcrum
• For a lever MA input arm/output arm
• There are 3 classes of levers first, second and
  third class
• For first class levers the fulcrum is located
  between the input force and the output force
• MA for first class levers is , lt or gt 1
• Examples seesaws, scissors, tongs, screwdriver

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• For second class levers, the output force is
  located between the input force and fulcrum
• MA is always gt1 for second class levers
• Example wheelbarrow
• For third class levers, the input force is
  located between the fulcrum and output force
• MA is always lt1 for third class levers
• Examples baseball bats, hockey sticks, golf
  clubs brooms

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• Def a wheel and axle consists of 2 disks or
  cylinders, each one with a different radius
• Example steering wheel
• To calculate MA for wheel and axle, divide the
  radius (or diameter) where the input force is
  exerted by the radius (or diameter) where the
  output force is exerted
• Def an inclined plane is a slanted surface along
  which a surface moves an object to a different
  elevation
• Example ramp in front of buildings
• The ideal MA for an inclined plane is the
  distance along the plane divided by its height

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• Def a wedge is V-shaped object whose sides are
  two inclined planes sloped toward each other
• Example flat head screwdriver
• A thin wedge of given length has a greater ideal
  MA than a thick wedge of the same length
• Def a screw is an inclined plane wrapped around
  a cylinder
• Screws with threads closer together have a
  greater ideal MA

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• Def a pulley consists of a rope that fits into a
  groove in a wheel
• The MA of a pulley or pulley system is equal to
  the number of rope sections supporting the load
  being lifted
• Def a fixed pulley is a wheel attached in a
  fixed location
• The ideal MA of a fixed pulley is always 1
• Def a movable pulley us attached to the object
  being moved
• The ideal MA of a movable pulley is 2

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• Def a pulley system is a combination of fixed
  and movable pulleys that operate together
• MA depends on pulley arrangement
• Def a compound machine is a combination of two
  or more simple machines that operate together
• Examples cars, washing machines, clocks