Bill Robertson May 18, 2006

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NSTA Web SeminarEnergy Stop Faking It!
Bill RobertsonMay 18, 2006
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Kinetic energy is

• Motion energy
• Energy of shape and/or position
• Usually difficult to calculate
• Energy associated with molecules

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Potential energy is

• Motion energy
• Energy of shape and/or position
• Usually difficult to calculate
• Energy associated with molecules

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Energy regularly goes from one form to another
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Which of the following describes the principle of
conservation of energy?

• Energy constantly changes form
• Conserving energy is necessary if were going to
  save the planet
• The total amount of energy in a closed system is
  always constant

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Work (net force) (distance object moves in the
direction of the force)
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Work done by a system subtracts energy from the
system
Work done on a system adds energy to the system
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If we believe in conservation of energy, then the
work you do on a system shows up as energy of the
components or work done by the system
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As long as we account for the work in, is our
ruler system a closed system (no energy in or
out)?

• Yes, because conservation of energy depends on
  having a closed system
• No, there is no such thing as a closed system
• Almost. There are always losses due to thermal
  energy, but those losses could be relatively
  small.

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Work in Work out
If we ignore heat losses due to friction, then we
can say that
Work done on the system work done by the
systemor
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F d F d

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Since d is larger than d , what do we know
about the forces?
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d is now smaller than d How do F and F
compare?
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You push down on a lever, moving your end four
times as far as the end with the rock moves. How
does the force exerted on the rock compare to the
force you exert?

• Theyre the same
• The force you exert is four times the force
  exerted on the rock
• The force you exert is one fourth the force
  exerted on the rock

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How does your pull in the previous slide compare
to the force exerted on the washer?

• The same
• Twice as large
• Half as large

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Sometimes all a simple machine does is change the
direction of the force you exert
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