Energy

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Energy

• According to Einstein, a counterpart to mass
• An enormously important but abstract concept
• Energy can be stored (coal, oil, a watch spring)
• Energy is something moving objects have
• How to deal with this idea???

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Work

• Easiest to start with the notion of work
• Work Force X Distance
• Lift a box from the floor, you apply a force to
  overcome gravity
• Multiply that force by the distance through which
  you apply the force and you calculate the amount
  of work accomplished

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Is this Work?
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Work

• Unit is the JOULE
• A Joule is a newton-meter

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Power

• The rate at which work is done
• Takes more power to run up the stairs than to
  walk up the stairs, but the energy consumed is
  the same in either case

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Power

• Unit is the WATT
• A Watt is a newton--meter per second
• Think of 100-Watt light bulb
• Bigger units are kilowatts and megawatts
• Utility sells energy in kilowatt-hours
• 1 KWh 1000 Joules/second times 3600 Seconds
  3.6 X 106 Joule

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Potential Energy

• If we lift an object from the floor into the air,
  it has the potential to do work for us
• This ability to do work is called POTENTIAL
  ENERGY
• Other forms of potential energy include the
  compression of a spring, the stored energy in
  coal or oil, the stored energy in a uranium
  nucleus

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Potential Energy

• Gravitational potential energy is simple to
  calculate
• Gravitational Potential Energy weight X height

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Gravitational Potential Energy

• Independent of Path to get there

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Kinetic Energy

• The energy of moving objects
• Kinetic Energy 1/2 Mass X Speed2

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Energy Conversion
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Energy Conversion
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Work-Energy Theorem

• Work done on an object can give the object either
  potential or kinetic energy or both
• If we do work on an object to lift it into the
  air, we give it potential energy
• If we do work on an object and set it into
  motion, we give it kinetic energy
• The work-energy theorem relates to the second case

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Work-Energy Theorem

• If we do work on an object and set it into motion
  without changing the objects potential energy,
  the work done appears as kinetic energy of the
  object

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Conservation of Energy

• Perhaps the most important discovery of the past
  two centuries
• In the absence of external work input or output,
  the energy of a system remains unchanged. Energy
  cannot be created or destroyed.
• Remember from Einstein, that mass is a form of
  energy

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Collisions

• Elastic Collisions conserve both momentum and
  kinetic energy
• Inelastic Collisions conserve momentum by energy
  is lost to heat

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Machines

• A device that multiplies forces by taking
  advantage of the definition or work and the
  conservation of energy
• Work input Work output
• Levers

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Machines
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Machines
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Efficiency
In many machines, some energy is lost due to
friction. This may be metal-on-metal (oil the
parts to reduce friction) or air resistance
(energy loss moves molecules in the air faster
giving them kinetic energy).
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Energy Sources

• For the earth, there are two energy sources, the
  sun and radioactive decay in the earths interior
• The earth receives about 1400 Joules/meter2 each
  second
• This is 1.4 kW per square meter
• Recover for use in plants (burn wood)
• Recover from wind

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Mans Need for Power

• Man can generate about 75 Watts to do work
• Domesticated Animal about 750 Watts
• Machines limited by size
• Power plants generate electricity in the hundreds
  of megawatt range

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Universal Gravitation (Newton)

• Every mass attracts every other mass with a force
  that is proportional to the product of the two
  masses divided by the square of the distance
  between the masses
• For distances, calculate from the CENTER OF MASS
• For the earth, that is at the center of the earth

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Universal Gravitation
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Acceleration Due to Gravity
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Inverse Square Law
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Inverse Square Law
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Weight and Weightlessness
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Tides
Stretch is about one meter high.