Chapter 8: energy

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Chapter 8 energy
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Work

• Work is done when you lift a load against earths
  gravity for example, when you lift a barbell or
  lift a chair to move it.
• Work is always done on an object. When you lift
  the barbell, you do work on the barbell.
• The work done on an object by an applied force is
  the product of the force and distance the object
  was moved.
• Work force x distance
• W Fd

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Two cases in which Work is done

• You do work on an object when you force it to
  move against the influence of an opposing force.
  Most of the time, when you move something against
  an opposing force, the opposing force is
  friction. If you pushed a desk across the
  classroom, you are doing work on the chair to
  move it against friction.
• Work is also done when you want to change the
  speed of an object. For instance, slowing a car
  down would be considering doing work on the car.

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W Fd

• Unit for Work is the Joule (named after James
  Joule) and is represented by J
• J Nm
• One joule (J) of work is done when a force of 1 N
  is exerted over a distance of 1 m
• Calculate the work done when a 30 N force pushes
  a cart 3.2 m

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• 1.) Calculate the work done in lifting a 200 N
  couch a distance of 1.3 m from a loading ramp.
• 2.) How much work is done when a 40 N force is
  applied to push a shopping cart a distance of 6
  m?
• 3.) Calculate how much work you would have to do
  to push your car (which has a mass of 254 kg) a
  distance of 3 meters.

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Power

• Power is the rate at which work is done.
• Power work done W
• time interval t
• Units for Power is the watt (W) after James Watt,
  the developer of the steam engine
• 1 watt (W) 1 joule per second (J/s)
• How much power is required to do 100 J of work in
  5 seconds?

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• 1.) How much power is needed to do 1200 J of work
  in 14 seconds?
• 2.) Calculate the power expended when a
  weightlifter exerts 110 J of work to lift a
  barbell in 3 seconds.
• 3.) Calculate the power of a car engine that can
  do 200,340 J of work in 2.5 seconds.
• 4.) If a forklift is replaced with a new forklift
  that has twice the power, how much greater a load
  can it lift in the same amount of time? If it
  lifts the same load, how much faster can it
  operate?

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• Decide if work is being done in each situation.
  If so, on what object (s) is the work being done?
•  
• 1.) A golfer hits a golf ball
• 2.) A basketball player makes a slam dunk
• 3.) You lift the couch cushion to look for extra
  change
• 4.) You hold the couch cushion above your head as
  you look for change
• 5.) You sit at your desk and write the answers
  down to these questions.

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Do Now 2/18/09

• Calculate the work done when a 45 N force pushes
  a chair a distance of 4 m
• An archer stretches her bowstring as she prepares
  to shoot at an arrow. Is work being done in this
  situation? If so, on what object is the work
  being done?
• Calculate the power expended for a car to do 345,
  000 J of work in 20 seconds.
• Do you do work on a boulder when you carry it a
  distance of 10 m?

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Energy

• Mechanical energy is the energy due to the
  movement of something it can be in the form of
  potential energy or kinetic energy.
• Energy that is stored until ready to be used is
  called potential energy, because in this stored
  state, it has the potential for doing work. The
  rubber band in a sling shot has potential energy
  because it has the potential of doing work when
  it is snapped. Energy is stored in the rubber
  band when it is stretched.
• The potential energy of a raised object (if you
  lift the couch to clean under it) is called
  gravitational potential energy.

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Gravitational Potential Energy

• Gravitational potential energy can be calculated
  using the equation
• PE mgh
• where PE is the gravitational potential energy,
  mg is the weight of the object in Newtons, and h
  is the height that the object is lifted.
• What is the gravitational potential energy of a
  50 kg boulder that is lifted 1 meter?

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• What is the gravitational potential energy of 1
  kg carton of milk that is lifted from the ground
  to a table that is 2 meters above the ground?
• Calculate the work needed to lift a 90 N block of
  ice a vertical distance of 3 m. How much PE
  (potential energy) does it have?
• An astronaut in full space gear climbs a vertical
  ladder on Earth. Later, the astronaut makes the
  same climb on the moon. In which location is
  there more gravitational potential energy?
  Explain

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Kinetic Energy

• When an object is lifted, stretched, or
  compressed, it has the potential to do work it
  stored what we called potential energy.
• When an object is in motion, it is also capable
  of doing work.
• The energy of an object in motion is called
  kinetic energy.
• The kinetic energy of an object depends on the
  mass of the object as well as its speed.
• KE ½ mv2

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Kinetic Energy

• Kinetic energy is also equal to the work that is
  done to put the object into motion.
• KE W
• KE Fd and KE ½ mv2
• Fd ½ mv2
• Calculate the kinetic energy of a 4 kg toy car
  that moves at 6 m/s.

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• 1.) Calculate the kinetic energy of the same toy
  car if it moves at twice the speed.
• 2.) Calculate the kinetic energy of a 2 kg rock
  that falls with a speed of 4 m/s.
• 3.) How much kinetic energy does a 0.8 kg arrow
  have if it is shot with an initial speed of 30
  m/s?

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Do Now 2/19/09

• 1.) State two reasons why a rock projected with a
  slingshot will go faster if the rubber is
  stretched an extra distance.
• 2.) If a mouse and an elephant both run with the
  same kinetic energy, can you say which is running
  faster? Explain in terms of the equation for
  kinetic energy.
• 3.) A hammer falls off a rooftop and strikes the
  ground with a certain KE. If it fell from a roof
  that was four times higher, how would its KE of
  impact compare? Its speed of impact?

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Law of Conservation of Energy

• Energy cannot be created or destroyed. It can be
  transformed from one form into another, but the
  total amount of energy never changes.

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Conservation of Energy

• Energy is always changing, but the total amount
  of energy in a given situation never changes.
• When you stretch the rubber band in a slingshot,
  potential energy is stored in the rubber band
  because it is stretched and has the potential to
  do work on the rock. Once the rubber band is
  released, the potential energy of the rubber band
  turns into kinetic energy.

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Machines

• Machines are designed to make work easier. They
  are designed to conserve (save) energy.
• There is always a work input and work output when
  dealing with a machine. The work that you put
  into the machine will equal the work that the
  machine puts out. Work input work output

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Machines and Mechanical Advantage

• Fd (input) Fd (output)
• The force that you exert on the jack multiplied
  by the distance that you move it will equal the
  force that the jack exerts on the car, multiplied
  by the distance that the car moves.
• The ratio of output force and input force for a
  machine is called the mechanical advantage. In
  the example with the car jack, if you input a
  force of 20 N, and the lever outputs a force of
  240 N, the ratio is
• 240 N 12
• 20N
• Therefore, the mechanical advantage 12
• When will the mechanical advantage be a large
  number?

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Do Now 2/20/09

• 1.) What is the law of conservation of energy?
• 2.) Describe the changes in energy that a
  pendulum encounters.
• 3.) When a machine has a greater output force
  than input force, is the value of the mechanical
  advantage large or small?

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Journal 2/20/09

• 1.) Is the creation in the video a machine? Why
  or why not?
• 2.) What characteristics of a machine does this
  creation have?
• 3.) Do you think this creation has a large or
  small mechanical advantage? Explain your answer.