Work, Energy, and Power

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Work, Energy, and Power
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Work

• Work is the product of the magnitude of the
  displacement and the component of the force
  acting in the direction of the displacement

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Work

• Most of the time F is in the direction of d so ?
  0 and cos 0 1 so
• Work is done by a force acting on a body!

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Work

• Symbol W
• Unit J, joule
• 1 J 1 Nm
• If force and displacement are in the
• Same directions, W
• Opposite directions, - W
• Perpendicular directions, W 0

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Work

• Work generally falls into 2 categories
• When you force something to move against the
  influence of an opposing force
• (push-ups gravitational force, walking
  friction, bow elastic force)
• When you change the speed of something
• A net force is present

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Total work causes a change in speed F constant
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Kinetic Energy Equation
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Work Energy Theorem or work done by a net
force or net work done on an object
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Work - graphing
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Work - graphing
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Work graphing with variable force
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Work with Varying Forces
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Hookes Law force required to stretch a spring
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Work in stretching a spring
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Work Energy Theorem with varying force
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Work and Energy

• Energy is the ability to do workOR
• Work causes changes in energyOR
• Work is a transfer of energy

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Work and Energy
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Power

• Power is the rate at which work is done or the
  rate at which energy is transformed.
• Symbol P
• Unit W, Watt
• 1W 1J/s

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Power
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Power and Work

• Example
• One liter of gas can do a specific amount of
  total work. However, the POWER produced when we
  burn the gas can be any amount depending on the
  rate of burn
• If
• 1L is burned over 30 min or 1800 sec, like a car
  50 units of power is produced
• 1L is burned in 1 sec, like a supersonic jet
  90,000 units of power is produced

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Energy - Types

• Mechanical Energy Energy due to position in a
  field force or energy due to movement
• Non-mechanical Energy Energy that does not fall
  into the above category

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Energy Flow Chart
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Mechanical Energy - Types

• Kinetic Energy, K Energy of a moving object
• Linear, KT or K ? center of mass moving (this
  chapter)
• Rotational, KR ? object rotating around center of
  mass (the chapter we cover after the AP exam)

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Mechanical Energy - Types

• Potential Energy, U Energy due to position in a
  field force
• Gravitational, Ug (this chapter)
• Elastic, Us (this chapter)
• Electric, UE (2nd Semester)
• Magnetic, UB (2nd Semester)
• You must choose a zero point for these

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Potential Energy Equations
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Conservative and Nonconservative Forces

• Conservative Force A force such that the work
  done on an object by the force does not depend on
  the path taken, rather it depends only on the
  initial and final positions (gravitational,
  elastic, electric)
• Nonconservative Force A force such that the work
  done on the object by the force does depend on
  the path taken (friction, air resistance, rocket
  propulsion). A lot of times these forces
  generate heat or sound which are non-mechanical
  energies.

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Work and U
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Energy Conservation

• The total energy is neither increased nor
  decreased in any process.
• Energy can, however, be transformed from one type
  to another AND transferred from one body to
  another, BUT, the total amount of energy in the
  process remains CONSTANT!

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Work Energy Principle Redefined

• So if energy is conserved we can write it this
  way using mechanical and non-mechanical energies

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Work Energy Principle Mechanical Energy
Conservation

• If we ignore nonconservative forces (friction and
  the such), the implication is that no
  non-mechanical energies are present (heat, sound,
  light, etc) therefore

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Mechanical Energy Conservation