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Energy, Work, and Simple Machines


Establish a coordinate system Draw a vector diagram. ... W = Fd For constant forces in the direction ... of one or more of the six simple machines ... – PowerPoint PPT presentation

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Title: Energy, Work, and Simple Machines

Chapter 10
  • Energy, Work, and Simple Machines

10.1 Energy and Work
  • Energy is the ability to produce change in itself
    or the environment.
  • Energy of Motion
  • vf2 vi2 2ad
  • Rearranging
  • vf2 - vi2 2ad
  • Substituting Newtons second law
  • vf2 - vi2 2Fd/m

  • Multiply both sides by ½ m
  • ½ mvf2 ½ mvi2 Fd
  • Kinetic energy
  • KE ½ mv2
  • Work-Energy Theorem
  • ?KE W
  • When work is done on an object, a change in
    kinetic energy results.

  • Work
  • Through the process of doing work, energy can
    transfer from the environment to the object and
    back again!
  • The equation for work is
  • W Fd
  • For constant forces in the direction of the
  • Unit of work is the joule (J)

  • Constant force at an angle
  • Work (angle between force and displacement).
  • W Fd cos ?

  • Power
  • The rate at which work is done
  • P W/t
  • Power is measured in watts (joule/second)

  • Sketch the problem.
  • Establish a coordinate system
  • Draw a vector diagram.
  • List known and unknowns.
  • Use the basic equation for work when a constant
    force is exerted in the same direction as the
    displacement. Or the equation W Fd cos ? which
    will work in all situations.

  • Use the work-energy theorem to determine the
    change in energy of the system.
  • Use work and time to find power.
  • Check your answer.

10.2 Machines
  • Simple and Compound Machines
  • The output work can never be greater than the
    input work
  • The machine simply aids in the transfer of
  • Mechanical Advantage
  • The force you exert on a machine is called the
    effort force Fe
  • The force exerted by the machine is called the
    resistance force Fr

  • The ratio of resistance force to effort force, is
    called the mechanical advantage.
  • Mechanical Advantage
  • MA Fr / Fe
  • This equation can be rewritten using the
    definition of work
  • Wo Wi or
  • Frdr Fede
  • Rearranging this gives
  • Fr/Fe de/dr

  • We know that the mechanical advantage is given by
    MA Fr/Fe
  • For an ideal machine, MA de/dr
  • Because this equation is characteristic of an
    ideal machine, the mechanical advantage is called
    the Ideal Mechanical Advantage
  • IMA de/dr

  • Efficiency
  • The efficiency of a machine is defined as the
    ratio of output work to input work.
  • Efficiency () Wo/Wi X 100
  • An ideal machine has equal output and input work
    and the efficiency is 100.
  • In terms of mechanical advantage and ideal
    machine advantage
  • Efficiency () (Fr/Fe)/(de/dr) X 100
  • Efficiency () MA / IMA X 100

  • Simple machines
  • Most simple machines are combinations of one or
    more of the six simple machines.
  • They are the lever, pulley, wheel and axel,
    inclined plane, wedge, and screw.
  • The IMA of all machines is the ratio of distances

  • Compound machines
  • A compound machine consists of two or more simple
    machines linked so that the resistance force of
    one machine becomes the effort force of the
  • The mechanical advantage of a compound machine is
    the product of the mechanical advantage of the
    simple machines it is made up of.
  • MA MA machine 1 X MA machine 2