Energy: Conservation, Work, and Transformations

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Energy Conservation, Work, and Transformations
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Part 1

• Law of Conservation of Energy

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Energy

• Energy is a scalar quantity that describes the
  ability to do work.
• There are many forms of energy.
• Energy is measured in Joules (J).

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The Law of Conservation of Energy

• Energy can not be created or destroyed.

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Roller Coaster Analysis
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• The ball has the greatest potential energy (PE
  mgh) right before the moment of release.
• It is not moving so it has no kinetic energy.
• Therefore, the potential energy at this point is
  the total energy available in the system.

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Energy Conservation

• Kinetic energy (KE 1/2mv2) at the lowest point
  of the roller coaster should be equal to the
  total energy of the system because there is no
  potential energy at that point (h0)
• Kinetic energy at the lowest point of the roller
  coaster should equal potential energy at the
  release point.

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Why dont the calculations match?

• Both air resistance and friction act on the ball,
  transforming some of the kinetic energy to
  thermal energy (heat).

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Conclusion

• Energy is never created or destroyed, but it does
  change its form.

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Part 2
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pleMachinesImages.jpg

• Work-Energy Theorem and Machines

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Work

• A transfer of energy that occurs when a force
  makes an object move in a direction parallel to
  the force.

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• FORMULA
• W F X d
• (Joules) (Newtons)
  (meters)
• Work Force X Distance

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EXAMPLEIf the force is 20N and the distance is
2m, Work F X dW 20N X 2mW 40 Joules
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Work-Energy Theorem

• The net work done on an object equals its change
  in kinetic energy.

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FORMULA

• W ?KE KEf KEi
• Work Change in Kinetic Energy
• Both Work and Energy are measured in Joules (J)

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Power

• amount of work done in a certain amount of time.

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• FORMULA
• P W ? t
• (Watts) (Joules)
  (seconds)
• Power Work Divided by Time

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EXAMPLEIf work is 20J and the time is
10s,Power W ? TP 20J ?10sP 2 Watts
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Machine

• A device that makes work easier.

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The Six Simple Machines
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Compound Machine

• A machine made up of more than one simple
  machine.
• The apple corer/peeler contains wedges, a wheel
  and axle, a lever, and a screw.

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_files/Image/apple-corer-peeler.jpg
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How Do Machines Make Work Easier?

• Since work equals force times distance, you can
  do the same amount of work by applying
• small force, long distance
• OR
• large force, short distance

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• FOR EXAMPLE
• If you have a force of 2N and a distance of 50m,
• WORK FORCE X DISTANCE
• WORK 50N X 2m
• WORK 100 JOULES
• If you have a force of 100N and a distance of 1m,
• WORK FORCE X DISTANCE
• WORK 100N X 1m
• WORK 100 JOULES

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• If you increase the distance,
• the amount of force needed to do the
• work is reduced.

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Mechanical Advantage

• Describes the factor by which the effort force is
  multiplied by the machine

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Pulley Station

• Increasing the number of pulleys and support
  strings increased the distance over which the
  force was applied.
• This caused an increase in mechanical advantage.

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How much of the input work is changed into useful
output work by the machine.
Efficiency
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• FORMULA
• W0 X 100
• WI
• EFFICIENCY WORK OUTPUT X 100
• WORK INPUT

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THE EFFICIENCY OF A MACHINE IS ALWAYS LESS THAN
100 BECAUSE SOME ENERGY IS CONVERTED TO HEAT DUE
TO FRICTION.
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EXAMPLEIf the input work is 100J and the
output work is 50J, EFFICIENCYWORK OUTPUT X
100 WORK INPUT
EFFICIENCY 50J X 100
100J EFFICIENCY 0.5J X 100 50
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• HOMEWORK
• PROBLEMS
• Copy the following problems and complete them for
  homework.

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1. When you and a friend move a 45kg couch to
another room, you exert a force of 75N over 5m.
How much work did you do? (DONT FORGET UNITS)
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2. How much power is required to push a car for
10 seconds if the amount of work done during that
time is 5500 Joules? (DONT FORGET UNITS)
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3. What is the efficiency of a machine which has
the input work of 20J and the output work of 5J?
(DONT FORGET UNITS)
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• 4. Initially, a 2 kg ball is flying through the
  air at a speed of 10 m/s. The speed of the ball
  is reduced to 8 m/s by air resistance. How much
  work is down on the ball by air resistance?

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Part 3

• Energy Transformations

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ages/EnergyTransformations.gif
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Types of Energy
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Station 1

• Food contains chemical energy
• We eat food to obtain energy.

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efoot_kitchen_witc/2008/02/valentines-da-1.html
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Station 2

• The rubber band will go flying if you stretch and
  then release it.
• This is a conversion of elastic potential energy
  to kinetic energy.

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/rubberband_3.htm
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Station 3

• The rubber ball has gravitational potential
  energy when you hold the ball.
• Gravitational PE is converted to kinetic energy
  when you drop it.

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89.JPG
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Station 4
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0000001/CrookesRadiometer_M.jpg
http//www.holymeatballs.org/2007/06/sl_gk_light_b
ulb_system.html

• Radiant energy from the light bulb is converted
  into kinetic energy to make the flags of the
  radiometer spin.
• The light bulb also converts electrical energy
  from the outlet to radiant and thermal energy.

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Station 5
http//www.amazon.com/Genecon-Hand-Generator/dp/B0
00701AZC
http//www.holymeatballs.org/2007/06/sl_gk_light_b
ulb_system.html

• The mechanical energy of spinning the hand
  generator handle is converted to electrical
  energy.
• The electrical energy is then converted to
  radiant and thermal energy, lighting the light
  bulb.

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Station 6

• Chemical energy in the battery is converted to
  electrical energy that travels down the wire and
  is converted to kinetic energy when the handle of
  the hand generator spins.

http//www.amazon.com/Genecon-Hand-Generator/dp/B0
00701AZC
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Station 7

• The chemical energy of the gasoline is converted
  to thermal energy and kinetic energy as the car
  moves around the track.

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Id11930
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Station 8

• Chemical Energy in the lemon is converted to
  electrical energy.

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ery.html
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Station 9

• Radiant energy is converted to electrical energy
  to power the calculator.

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ex7.html
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Station 10

• The roller coasters gravitational potential
  energy at the top of the hill is converted to
  kinetic energy on the way down.

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