# Energy

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```					 Work and Energy

More relationships between
forces, time and motion
What is energy?
   Central concept in science
   Energy is the mover and changer of matter
   Isaac Newton didn’t think about things in terms
of energy!
   We think about energy mostly in terms of what
happens when it’s transformed
   Energy is defined as the capacity to do work
•   Energy has units of Joules
Classifying Energy
Energy

Non-Mechanical
Mechanical Energy
Energy

Potential Energy                          Kinetic Energy
Heat
(Energy due to position)                   (Energy of motion)

Gravitational                                          Light

Elastic                                            Nuclear

Chemical
Conservation of Energy
   Energy is neither created nor destroyed
unless there is external work input or
output on the system
   Instead energy is transformed from one
form to another
   Examples:
• Matches
• Car Engines
BIG IDEA: What is the system?
   We saw this before when we talked
   Systems are either
• Open
• Energy is coming in or leaving, or work is being
done on the system
• Closed
• Energy is not entering or leaving the system
Conservation of Energy
   BUT remember E=mc2
• Mass and energy can be converted into each
other
•   Only where there are nuclear reactions going
on
• Fission
• Fusion
Potential Energy
   Potential energy is energy stored as the
result of an object’s position
• Elastic—rubber band example
• Gravitational—dropping an apple
   Units of P.E. are J
Potential Energy and Work
   Work to lift an object to a height h at constant
velocity
•   Applied force and force of gravity are equal and
opposite

Work  Fappl  h  Fgrav  h  P.E.
   Work done on the object is equal to the
potential energy of the object
•   Work = P.E.
Potential Energy

P.E.   Fgrav  h  mg  h  mgh
h  height in m
g  10 m/s 
2
Potential Energy

P.E.   Fgrav  h  mg  h  mgh
h  height
g  10 m/s  2
Potential Energy
   Understanding the equation

P.E.  Fgrav  h  mg  h  mgh
•   As height is doubled, PE is doubled
•   As mass is doubled, PE is doubled
Kinetic Energy
   Kinetic energy is the energy of motion, where v is
velocity
KE  mv 1
2
2

   KE is a scalar

   If mass is doubled, KE is doubled
   If velocity is doubled, KE is multiplied by 4

   Units of KE are Kgm2/s2=Joule
Conservation of Energy
Energy converts between potential and
kinetic in a pendulum if no friction
Ideal vs. Real World
Elastic Collisions
No energy losses

Inelastic Collisions
Energy losses
A real life inelastic collision
   Watch the bear!
   Inelastic in the extreme

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 views: 8 posted: 12/11/2011 language: pages: 15