Work and Energy

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Work and Energy

• PVHS Physics
• 2009

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Objectives

• What is energy?
• What is work?
• How do you determine the amount of work done on
  an object?
• What is the difference between work and power?
• How do you calculate power?
• What are potential and kinetic energy?
• How is energy related to work?
• What is conservation of energy?
• What is a closed system?

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A Little Review

• In physics, work is defined as a force acting
  upon an object to cause a displacement.
• Three key words Force, displacement, and cause
• For work to be done There must be a force and
  that force must cause a displacement

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A Little Review

• Work equals Force times Displacement
• Force and Displacement are vectors
• Work is accomplished only by the component of the
  force vector in the direction of the displacement

F
Fy
Q
Fx
y
d
x
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More on Work
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Daily Science Question

• On which ramp will the soapbox racer be moving
  the fastest when it reaches the bottom?

A
B
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What is Energy?
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Kinetic Energy

• Kinetic Energy is energy stored in motion

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Kinetic Energy

• Kinetic Energy is energy stored in motion
• Any object in motion, that has velocity,
  possesses Kinetic Energy
• KE related to the square of the objects velocity

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Kinetic Energy

• Math Definition
• Energy is not a vectorbut
• You can add or take away energy
• The change in energy can be positive or negative

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Relating Work to Kinetic Energy

• Kinetic Energy

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Work-Energy Theorem

• The Work (W) done on an object is equal to a
  change in its Kinetic Energy
• W DKE
• Work transfers Energy into or out of a system

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• Can forces do work without causing a change in KE
  or velocity?
• Where does the energy go if the Net work is zero?

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Special Cases

• Friction
• Opposes motion always negative work
• Reduces the increase in kinetic energy
• Energy is lost for good (stored as Einternal)
• Gravity
• Moving against gravity stores energy
• Moving with gravity releases stored energy
• Elastic Forces (springs, rubber bands)
• Like gravity Elastic forces store and later
  release energy

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Potential Energy

• Potential energy is just stored energy
• Abbreviated as PE
• Usually based on an objects relative position
• Gravitational Potential Energy
• Higher objects have more potential energy than
  lower objects
• Elastic Potential Energy
• Energy stored in a stretched or compressed object

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Other Conventions

• In the book, on websites, and in other references
  you might see
• KE energy stored in motion
• PE energy stored by gravity
• Peel energy stored elastically

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Power

• Work equals force times displacement
• F x d can equal a F x d
• 100N x 5m 5N x 100m
• Which took more effort how can you
  differentiate?
• Power
• Relates time and work (or any energy transfer)
• Power is the rate at which energy is transferred

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Power

• Remember Power actually relates energy transfer
  to time
• Work is not the only way to transfer energy
• Heat
• Radiation

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Sample Problems

• A 70kg sprinter crosses the finish line moving 9
  m/s. How much KE does the sprinter have?
• You apply a 10 N force to a frictionless cart
  with a mass of 5kg for a distance of 20 meters.
• How much work did you do?
• How much KE does the cart have after 20 meters?
• How fast is the cart moving after 20 meters?

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Sample Problems

• It takes you 1500 N-m of work to drag a chair 10m
  across a carpeted floor. How much energy was lost
  due to friction?
• You drop a 2kg book and it falls 1m to the floor.
  How much KE does the book have when it hits? How
  fast?
• How much power was used to lift a 100kg crate 5m
  if it took 6 seconds?

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Objectives

• What is work?
• How do you determine the amount of work done on
  an object?
• What is the difference between work and power?
• How do you calculate power?
• What are potential and kinetic energy?
• How is energy related to work?
• What is conservation of energy?
• What is a closed system?

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Any Questions?