Chapter 8: Work, Power, and Energy

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Chapter 8 Work, Power, and Energy
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Work and Energy

• Energy
• The ability to do work.
• Work
• A force causes an object to be moved through a
  distance.
• Transfer of Energy

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Work

• Work is done when energy is transferred from one
  system to another.
• An object is lifted (gravitational)
• A rubber band is stretched (elastic)
• Water is heated (thermal)

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Work

• Work Force x Distance
• The units on work are Joules (J).
• The quantity force x distance is equal to the
  change in energy.
• If a force F acts over a distance d, and F is
  parallel to d, then the work done by F is the
  product of force and distance W Fd.

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Work

• The greater the load, or the further we move it,
  the more work we do
• The amount of work done depends on
• Force exerted
• Distance over which the force is exerted
• The more the work done, the greater the amount of
  energy gained by the object.

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Work Force x Distance
Which cart required a greater input of energy in
order to lift?
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Work

• A weightlifter does work when he lifts a barbell,
  but as he holds the barbell still above his head,
  he does no work.
• If you push on a wall, but it doesnt move, did
  you do any work? Why or why not?

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Work Example Problems

• How much work is done if a 10N chair is lifted
  6m?
• How much work is done if I move a 5N book 5m?
• How much work is done if I push on a wall with a
  force of 50N but the wall does not move?

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Work Example Problems

• You did 50J of work to lift a 25N object. How
  high did you lift the object?
• You did 300J of work to move a 1000N piano. How
  far did you move the piano?
• You lift a 500N object 3m. How much energy did
  you put into the object? What will happen to the
  energy contained in the object when you drop it?

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Energy

• The ability to do work
• Two main types
• Potentialstored energy
• Kineticenergy of motion

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Energy transformation of a Pendulum
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Energy transformation on a roller coaster

• At what point does a roller coaster have the most
  potential energy?
• What happens to the potential energy as the
  roller coaster rolls down hill?
• Where did the potential energy initially come
  from?

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Potential Energy

• Stored Energy
• Energy of position
• Types of potential energy
• Elastic
• Chemical
• Gravitational
• The amount of potential energy possessed by an
  object is equal to the amount of work done in
  supplying the potential energy.

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Potential Energy

• Potential Energy mass x gravity x height
• PE mgh
• m mass (kilograms)
• g acceleration due to gravity (9.8 m/s2)
• h height (m)
• PE Joules (J)

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Potential Energy Examples

• A 5kg object is lifted 10m, how much potential
  energy does it now possess?
• A 1000N wrecking ball is lifted to a height of
  25m.
• How much potential energy does it possess?
• How much work was done in lifting it to this
  height?

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Potential Energy Examples

• How does the wrecking ball in Example 2 compare
  to a wrecking ball that sits on the ground?
• Why is this wrecking ball more useful to
  construction workers?

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Potential Energy Examples

• At what part of the roller coaster ride does the
  car gain potential energy?
• At what part of the roller coaster ride does the
  car release potential energy?
• Where does this energy go?

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Potential Energy Examples

• A car is lifted a certain distance in a service
  station and, therefore, has potential energy
  relative to the floor. If it were lifted twice
  as high, how much potential energy would it have?
• Two cars are lifted to the same elevation in a
  service station. If one car is twice as massive
  as the other, how do their potential energies
  compare?

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Kinetic Energy

• Energy of motion
• KE 1/2mv2
• An object must be moving in order to have kinetic
  energy.
• Depends on two factors
• Mass
• Velocity

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The Law of Conservation of Energy

• Energy cannot be created of destroyed

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Law of Conservation of Energy

• In reality, does a roller coasters final kinetic
  energy equal the initial potential energy? Why
  or why not?
• Tennis ball demonstration

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Law of Conservation of Energy

• In reality, energy is lost to friction, wind
  resistance, and heat.
• Notice the difference here Energy is LOST, but
  not DESTROYED.
• Once the energy is lost, where does it go?

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Kinetic Energy Examples

• A 1000kg roller coaster is on top of a 50m hill.
• What are the KE and PE of the car at the top of
  the hill?
• What are the KE and PE of the car halfway down
  the hill?
• What are the KE and PE of the car at the bottom
  of the hill?
• Ideally, what is the velocity of the car at the
  bottom of the hill?

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Power

• The rate at which work is done
• Power work divided by time
• P W/t
• Because W Fd, P Fd/t
• The metric unit of power is the watt.
• The English unit of power is the horsepower
• One horsepower equals 750 Watts

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Power Examples

• Two people each weigh 200N and climb a flight of
  stairs that raises them 10m vertically.
• Who did more work?
• If person A climbed the stairs in 20 seconds, and
  person B climbed the stairs in 40 seconds, who
  generated more power?

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Power Examples

• A car that can accelerate from 0 to 60mph in 4
  seconds will have _________ power than a car that
  can accomplish the same acceleration in 10
  seconds.
• Suppose a 40 horsepower engine can accelerate
  from 0-60mph in 16 seconds. A car with four
  times more power could accomplish the same feat
  in how many seconds?

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Energy

• Have you ever noticed the electric meter on the
  side of your house? What unit does the meter use
  to measure the amount of energy you use?
• To determine how much you pay for electricity,
  the power company measures how much power you
  use, and multiplies it by the number of hours for
  which you use it. The number of kilowatt-hours
  is then multiplied by whatever the current rate
  is, and that is what you pay for electricity that
  month.

P W/t Pt W Pt energy used
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Power Examples

• A person does 500J of work in 10 seconds. How
  much power did they generate?
• A person does 50J of work to move a box 7m in 4
  seconds. How much power did they generate?
• In 5 seconds, you lift a 5N box 4m off the
  ground. How much power did you generate?

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Power Examples

• Person A weighs 230N and climbs 6m in 10 seconds.
  Person B weighs 100N and climbs 10m in 10
  seconds. Who generated more power?
• You exert 50N of force to push a box 4m in 15
  seconds. How much power did you generate? How
  much horsepower did you generate?

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Power Examples

• You run a 60 watt light bulb for 10 hours. If
  the electric company charges 0.05 per
  kilowatt-hour, how much did you pay to run the
  lamp for 10 hours? Remember, 60W 0.060kW.
• Georgia Power sells you ___________ NOT power.