How Machines Do Work?

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Chapter 4.2

• How Machines Do Work?

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Input and Output Work
- How Machines Do Work

• The amount of input work done by the gardener
  equals the amount of output work done by the
  shovel.

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Input Output work

• Input force the FORCE you exert on the machine.
• Output force the FORCE the machine exerts ON
  THE OBJECT

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Calculating Input Output work

• Input work
• Input force x Input distance
• Output work
• Output force x Output distance
• When you use a machine, the amount of output work
  can never be greater than the amount of input
  work.

5
What Is a Machine?
- How Machines Do Work

• A machine makes work easier by changing at least
  one of three factors. A machine may change the
  amount of force you exert, the distance over
  which you exert your force, or the direction in
  which you exert your force.

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Mechanical Advantage

• If you compare the input force to the output
  force, you can find the advantage of using a
  machine.
• A machines mechanical advantage is the number of
  times a machine increases a force exerted on it.

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Calculating Mechanical advantage

• Mechanical advantage Output force
• input force
• Examples
• A machine that increases force output force is
  greater than input force.
• ex. a can opener

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• For a machine that increases distance the output
  force is less than the input force.
• Ex. A wooden spoon vs. a regular spoon.
• For a machine that changes direction the input
  force will equal the output force
• Ex. A weight pulley machine
• The mechanical advantage of these types of
  machines will always be 1.

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Mechanical Advantage
- How Machines Do Work

• Interpreting Data
• If an 80-N input force is exerted on Ramp 2, what
  is the output force?

• 400 N

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Efficiency of Machines

• So far you learned that input work is equal to
  output work. This does not happen is real life.
• WHY??? FRICTION
• In every machine, some work is wasted overcoming
  friction. Force must be applied to overcome the
  friction of the machine.

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Efficiency of machines

• The efficiency of a machine compares the output
  work to the input work.
• Efficiency is calculated as a percentage ()
• Efficiency output work
• input work

X 100
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Calculating Efficiency
- How Machines Do Work

• You do 250,000 J of work to cut a lawn with a
  hand mower. If the work done by the mower is
  200,000 J, what is the efficiency of the lawn
  mower?
• Read and Understand
• What information have you been given?
• Input Work (Winput) 250,000 J
• Output Work (Woutput) 200,000 J

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Calculating Efficiency

• You do 250,000 J of work to cut a lawn with a
  hand mower. If the work done by the mower is
  200,000 J, what is the efficiency of the lawn
  mower?
• Plan and Solve
• What quantity are you trying to calculate?
• The efficiency of the lawn mower __
• What formula contains the given quantities and
  the unknown quantity?
• Efficiency Output work/Input work X 100
• Perform the calculation.
• Efficiency 200,000 J/250,000 J X 100
• Efficiency 0.8 X 100 80
• The efficiency of the lawn mower is 80 percent.

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Real Ideal Machines

• Real machines
• Efficiency is always less than 100
• Ideal machines
• Efficiency is always 100
• ( these machines do not exist)