Simple Machines

1
Simple Machines

• Chapter 14

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What do All These Things Have in Common?
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Questions

• What is the function of the machines?
• How many moving parts do they have?
• How do the parts work together?
• How does this machine make work easier?

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Machines change the Way Force is Applied

• A machine is any device that helps people do
  work.
• They change the way in which work is done.

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How Could we Move this Couch Easier?
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Machines

• If machines do not reduce the amount of work
  required, how do they help people do work?
  Machines make work easier by changing
• The size of the force needed to do the work and
  the distance over which the force is applied.
• The direction in which the force is exerted.

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Have you ever tried to open a door by turning the
doorknobs shaft instead of the handle?

• The doorknob allows you
• to use less force to turn
• the handle instead of the
• shaft.

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Machines Changing Size Direction

• The doorknob is a machine. It allows you to
  apply a smaller force over a greater distance.
• The boy using the rake
• is a machine. He moves
• his hands a short distance
• to move the end of the
• rake a large distance,
• Allowing him to rake more
• Leaves.

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Machines

• Input Force is the force exerted on a machine.
• Output Force is the force that a machine exerts
  on an object.
• Picture on page 450

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Changing Direction

• Machines also help you work by changing the
  direction of force.
• You pull down on a rope
• and the flag moves up.

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Mechanical Advantage

• When machines help you work, there is an
  advantageor benefit to using them?
• Mechanical Output Force
• Advantage Input Force

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Mechanical Advantage

• For machines that allow you to apply less force
  over a greater distance such as a doorknob the
  output force is greater than the input force.
• Therefore, the mechanical advantage of this type
  of machine is greater than 1.
• For example, if the input force is 10 newtons and
  the output force is 40 newtons, the mechanical
  advantage is
• 40 N / 10 N
• MA 4

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Mechanical Advantage

• Sometimes changing the direction of the force is
  more useful than decreasing the force or
  distance.
• Therefore, the mechanical advantage of the
  machine is 1.

14
Practice Problem

• The output force of a machine is
• 600 N and the input force is 200 N. What is
  the mechanical advantage of the machine?
• MA Output force 600 N 3
• Input Force 200 N

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Work Transfers Energy

• Machines transfer energy to objects on which they
  do work!

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Energy

• When you lift an object, you transfer energy to
  it in the form of gravitational potential energy
  that is, potential energy caused by gravity

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Work

• When you use a machine to do work, there is
  always an exchange, tradeoff, between the force
  you use to do the work and the distance over
  which you apply that force.

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Mountain ClimbingPage 453

• Short Route Climbing straight up covers a
  shorter distance but more force must be applied
  against gravity
• Long Route Climbing a gentle slope, the climber
  covers more distance but uses less force.

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Efficiency

• The work you put into a machine will always be
  greater than the work done by the machine. Some
  input work is always lost in overcoming friction.
• Efficiency () Output work x 100
• Input work

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Efficiency and Energy

• Work transfers energy and that machines make work
  easier.

• Work transfers energy and that machines make work
  easier.

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Energy Efficiency

• Many appliances come with energy guides that can
  help a buyer compare the energy efficiency of
  different models.
• A washing machine with the highest energy rating
  may not always save the most energy, because
  users may have to run those machines more often.

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Review Questions

• In what ways can a machine change a force?
• By changing the size of a force, changing its
  direction, or both
• How is a machines efficiency calculated?
• Efficiency () Output work x 100
• Input work