Work%20and%20Energy%20Unit

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Work and Energy Unit

• Chapter 9

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Energy

• The ability to do work or cause change
• Can be transferred into other forms (energy flow)
• Is conserved (can neither be created nor
  destroyed)
• SI Unit is Joules
• Anything with energy can produce a force that is
  capable acting over a distance

• LT 1
• I can define energy

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Work

• Force times distance the force is applied (W
  Fd)
• When work is done, energy is transferred, stored
  or used (a change occurs)
• SI Unit is Joules 
• Work is done by forces
• The object must move for work to be done

• LT 2
• I can define work.

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Power

• The rate at which energy is transferred or work
  is done (work per second)
• SI Unit is Watts (Joules/second)
• The faster the energy is used, the greater the
  power
• More powerful if
• more work is done in same time
• same work is done in less time 

• LT 5
• I can define power and its relationship to energy.

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Work

• Positive work is work done by a force acting in
  the direction of the displacement (or motion).
• (example force applied by engine to wheels of a
  car)
• Negative work is work done by force acting in the
  opposite direction of the displacement (or
  motion)
• (example Friction)

• LT 3
• I can identify the difference between positive
  and negative work.

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Work

• Another way of looking at this
• Positive work adds energy to the system
• Negative work takes away energy from the system

• LT 3
• I can identify the difference between positive
  and negative work.

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6.1 Work force distance

• a) Did the weightlifter do work on the barbell
  and weights?
• b) Is the weightlifter currently doing work on
  the barbell and weights?
• c) Explain two ways that the work done by the
  weightlifter might be increased.
• 1.
• 2.

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9.1 Work force distance

• Did the weightlifter do work on the barbell and
  weights?
• Yes, when he first lifted them above his head.
• Is the weightlifter currently doing work on the
  barbell and weights?
• No, the barbell and weights are not moving.
• Explain two ways that the work done by the
  weightlifter might be increased.

1. Increase the weight on the ends of the barbell
2. Increase the distance over which the weightlifter
   pushes the barbell and weights.

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9.1 Work
While the weight lifter is holding a barbell over
his head, he may get really tired, but he does no
work on the barbell. Work may be done on the
muscles by stretching and squeezing them, but
this work is not done on the barbell. When the
weight lifter raises the barbell, he is doing
work on it.
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9.1 Work
Work has the same units as energy Joules
Newton x meter
J N x m

• One joule (J) of work is done when a force of 1 N
  is exerted over a distance of 1 m (lifting an
  apple over your head).

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What happens to KE and TME when the brakes are
applied? What work is being done?
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Watch the transfer of KE and PE.
What happens to the PE when the skier moves down
the hill? What happens to the KE and TME when the
skier travels over the unpacked snow? What work
is done?
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Horsepower
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9.2 Power

• Jet engine vs. lawn mower engine
• Both receive ½ gallon of fuel (same energy, same
  work)
• A high-power jet engine does work rapidly, uses ½
  gallon in 1 second.
• The low-powered lawn mower engine does work
  slowly, using ½ gallon in 30 minutes.

vs.
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P w/t
9.2 Power
Power is the rate at which work is done.
The unit of power is the joule per second, also
known as the watt. One watt (W) of power is
expended when one joule of work is done in one
second. One kilowatt (kW) equals 1000 watts.
One megawatt (MW) equals one million watts.
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Power

• When you run 3 km rather than walk, you use the
  energy more quickly because your body demands
  more energy per unit time.
• When you compare the amount of energy required to
  operate an electric dryer vs. a laptop computer,
  the electric dryer demands more energy per unit
  time.
• More energy per unit time means more power is
  required!

Needs 5500 J/s
Needs 50 J/s
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Power
100 W incandescent light bulb How much
electrical energy per second? 100 joules per
second.
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Power vs. Work

• When carrying a load up some stairs, you do the
  same amount of work whether you walk or run up
  the stairs.
• Whether you walk 3 km or run 3 km, you do the
  same amount of work (your weight x distance),
  burn roughly the same amount of calories, and use
  the same amount of energy.
• So what is power?

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Power

• Consider a person climbing stairs.
• Name two ways that the person can double their
  power when moving.
• Do twice the work in the same amount of time
  (climb a second flight of stairs in the same
  time)
• Do the same amount of work in half the time
  (climb one flight of stairs in half the time).

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9.2 Power
The three main engines of the space shuttle can
develop 33,000 MW of power when fuel is burned at
the enormous rate of 3400 kg/s.
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9.2 Power

• think!
• 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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9.2 Power

• think!
• 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?
• Answer
• The forklift that delivers twice the power will
  lift twice the load in the same time, or the same
  load in half the time.

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Watch the transfer of KE and PE.
What happens to the PE when the skier moves down
the hill? What happens to the KE and TME when the
skier travels over the unpacked snow? What work
is done?
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9.1 Work
When the object moves.
When is work done on an object? When is work not
done on an object?
When the object does not move.
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Kinetic Energy

• The energy of motion
• KE ½m x v2
• Different forms of KE (mechanical, electrical,
  thermal, electromagnetic or light)

• What is kinetic energy?
• What are the forms of KE?

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Kinetic Energy
KE increases with mass
KE increases with speed
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WIND ENERGY

• Atmospheric pressure differences cause air
  particles to move.

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SOUND ENERGY

• Energy caused by compression of air particles.

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ELECTRICAL ENERGY

• Energy of moving charged particles.

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THERMAL ENERGY

• The energy of moving and vibrating molecules
• Sometimes called heat.

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LIGHT or RADIANT ENERGY

• Energy that travels in waves as electromagnetic
  radiation and/or as photons.

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9.5 Kinetic Energy
When you throw a ball, you do work on it to give
it speed as it leaves your hand. The moving ball
can then hit something and push it, doing work on
what it hits.
WORK
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9.5 Kinetic Energy

• If the speed of an object is doubled, its kinetic
  energy is quadrupled (22 4).
• It takes four times the work to double the speed.
  
• An object moving twice as fast takes four times
  as much work to stop and will take four times as
  much distance to stop.

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Kinetic Energy

• How does KE increase or decrease?
•   Increase or decrease the velocity or the
  mass!!!!
• Double the velocity, Quadruple the KE!!!!!
•  
• Prove it Calculate the KE of a 2500 kg car
  traveling at 20 m/s and at 40 m/s
• KE at 20 m/s KE at 40 m/s
• (500,000 J) (2,000,000 J)
•  

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Kinetic Energy

• More mass, same speed, more KE.
• Double the mass, double the KE
• Prove it Calculate the KE of a 100 kg cart and
  a 200 kg cart, each traveling at 15 m/s
• 100 kg cart at 15 m/s 200 kg cart at 15 m/s
• (11,250 J) (22,500 J)
•  
•  

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Potential Energy

• Stored energy or the energy of position
• Gravitational PE is based on height and mass
• Gravitational PE is mass x gravity x height (GPE
  mgh)
• Increases in height cause increases in stored
  energy

• What is potential energy?
• How does GPE change?

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9.4 Potential Energy

• Gravitational Potential Energy

• Energy is stored in an object as the result of
  increasing its height.
• Work is required to elevate objects against
  Earths gravity.
• Example Water in an elevated reservoir and the
  raised ram of a pile driver have gravitational
  potential energy.

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9.4 Potential Energy
The amount of gravitational potential energy
possessed by an elevated object is equal to the
work done against gravity to lift it. PE
mgh What is the gravitational PE of a 10.0 kg
object at 4.00 m above the ground? mg is weight
(in newtons) mass (kg) x gravity (m/s2) 10 kg
x 9.8 m/s2 x 4 m 392 J
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9.4 Potential Energy

• The potential energy of the 100-N boulder with
  respect to the ground below is 200 J in each
  case.
• The boulder is lifted with 100 N of force.

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9.4 Potential Energy

• The potential energy of the 100-N boulder with
  respect to the ground below is 200 J in each
  case.
• The boulder is lifted with 100 N of force.
• The boulder is pushed up the 4-m incline with 50
  N of force.

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9.4 Potential Energy

• The potential energy of the 100-N boulder with
  respect to the ground below is 200 J in each
  case.
• The boulder is lifted with 100 N of force.
• The boulder is pushed up the 4-m incline with 50
  N of force.
• The boulder is lifted with 100 N of force up each
  0.5-m stair.

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9.4 Potential Energy

• think!
• You lift a 100-N boulder 1 m.
• a. How much work is done on the boulder?
• b. What power is expended if you lift the boulder
  in a time of 2 s?
• c. What is the gravitational potential energy of
  the boulder in the lifted position?

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Other forms of PE

• Other forms of PE (Chemical PE, Elastic PE,
  Electric PE, Magnetic PE, Nuclear PE)
• Changes in position in a force field changes the
  PE (gravitational fields, magnetic fields and
  electric fields)

• What are the forms of potential energy?

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9.4 Potential Energy

• Elastic Potential Energypotential to do work

• Energy stored in a stretched or compressed spring
  or material.
• When a bow is drawn back, energy is stored and
  the bow can do work on the arrow.
• These types of potential energy are elastic
  potential energy.

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CHEMICAL POTENTIAL ENERGY

• Energy due to the bond position between molecules
  (stored during bonding).
• Potential chemical energy is released from
  chemical reactions (burning, for example).
• Fuels, Food, Batteries, for example.

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9.4 Potential Energy
Name three examples of potential energy.
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Difference between kinetic energy and potential
energy

• Kinetic energy  
• The energy of motion

• Potential energy
• The energy of position or stored energy

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Mechanical Energy

• The sum of the KE and PE in a system (total ME
  KE PE)
• Describes energy associated with the motion of
  objects
• The KE and GPE are conserved for moving objects
  (neglecting friction, drag, vibrations and sound)

• What is mechanical energy?

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Mechanical Energy PE KE

• The total mechanical energy 100 J

100 J 100 J PE 0 J KE
100 J 50 J PE 50 J KE
100 J 0 J PE 100 J KE
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Watch how KE and gravitational PE transform
Where is the KE at the maximum? Where is the PE
at the maximum? How is PE stored?
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Watch the change in height vs. the change in
speed!
How does the change in height affect KE and PE?
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Slides showing transformation of KE and PE

• Source http//www.physicsclassroom.com/mmedia/ind
  ex.cfm

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Same work, more force, less displacement (from
left to right)
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Non-Mechanical Energy

• Energy not associated with the motion of objects
• Typical examples are vibrations, sound and heat
• Referred to as dissipated energy or waste energy
• Can be observed at the molecular level
• Path of energy transfer that reduces the KE of
  the object

• What is non-mechanical energy?

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• Indicate where
• KE is at a minimum and maximum
• GPE is at a minimum and maximum
• The speed is greatest
• The speed is least
• Energy is being stored and released

Positions 1 and 5 are at the same height
1. Explain how energy transforms and is
conserved as the pendulum swings back and forth
2. What happens as the KE increases? 3. What
happens as the GPE increases?
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KE min
KE min
PE max
PE max
V 0 m/s
V 0 m/s
transformation of PE to KE (release)
transformation of KE to PE (storage)
KE max
PE min
V maximum
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Analyzing KE and PE
farthest
Distance (from motion detector)
closest
time
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Work Energy Theorem

• Work done changes the energy. If a car has
  34,000 J of KE, 34,000 J of work was done on the
  car to speed it up, and braking will require
  34,000 J of negative work due to friction to
  bring the car to rest

• What is the relationship between work and kinetic
  energy (work-energy theorem)?

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9.6 Work-Energy Theorem

• Due to friction, energy is transferred both into
  the floor and into the tire when the bicycle
  skids to a stop.
• An infrared camera reveals the heated tire track
  on the floor.

http//www.batesville.k12.in.us/physics/phynet/mec
hanics/energy/braking_distance.htm
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9.6 Work-Energy Theorem

• Due to friction, energy is transferred both into
  the floor and into the tire when the bicycle
  skids to a stop.
• An infrared camera reveals the heated tire track
  on the floor.
• The warmth of the tire is also revealed.

kinetic energy is transformed into thermal
energy, sound and vibrations, which represent
work done to slow the bike (Fd)
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9.6 Work-Energy Theorem

• The work-energy theorem states that whenever work
  is done, energy changes.

Work ?KE Work equals the change in kinetic
energy.
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Calculating Stopping Distance

• Fd ½ mv2
• What is the stopping distance for a 650 kg car
  that is traveling 5 m/s if 4,500 N of braking
  force is applied?
• d ½ mv2
• F
• d 1.8 m
• Calculate the stopping distance for the same car
  that travels at 10 m/s.
• 7.2 m.

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Calculating Stopping Distance

• Calculate the stopping distance for the same car
  that travels at 10 m/s.
• 7.2 m.
• How does this stopping distance compare with the
  stopping distance at 5 m/s?
• It is four times greater!
• Double the speed, quadruple the stopping
  distance.

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Calculate Stopping Distance

• Fd ½ mv2
• -Calculate the difference in stopping distance
  for a car that travels at 30 km/h and the same
  car that travels 60 km/h. Assume that the mass
  of the car is 800 kg and the braking force is
  5000 N. Show your work and analyze your results.
  (Note you must first convert km/h to m/s)
• How does speed influence stopping distance?

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9.6 Work-Energy Theorem
A car moving at twice the speed of another has
four times as much kinetic energy, and will
require four times as much work to stop. The
frictional force is nearly the same for both
cars, so the faster one takes four times as much
distance to stop. Kinetic energy depends on
speed squared.
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9.6 Work-Energy Theorem
Typical stopping distances for cars equipped with
antilock brakes traveling at various speeds. The
work done to stop the car is friction force
distance of slide.
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9.6 Work-Energy Theorem
Typical stopping distances for cars equipped with
antilock brakes traveling at various speeds. The
work done to stop the car is friction force
distance of slide.
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9.6 Work-Energy Theorem
Typical stopping distances for cars equipped with
antilock brakes traveling at various speeds. The
work done to stop the car is friction force
distance of slide.
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9.6 Work-Energy Theorem

• think!
• When the brakes of a car are locked, the car
  skids to a stop. How much farther will the car
  skid if its moving 3 times as fast?

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9.6 Work-Energy Theorem

• think!
• When the brakes of a car are locked, the car
  skids to a stop. How much farther will the car
  skid if its moving 3 times as fast?
• Answer
• Nine times farther. The car has nine times as
  much kinetic energy when it travels three times
  as fast

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9.6 Work-Energy Theorem
For moving objects such as cars The more kinetic
energy it has, the more work is required to stop
it. Twice as much kinetic energy means twice as
much work. Brakes do work on wheels (you do
work by pushing the brake pedal). When a car
brakes, the work is the friction force (supplied
by the brakes) multiplied by the distance over
which the friction force acts. KE is transformed
by work (friction) into thermal energy, sound
energy and larger-scale vibrations.
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9.7 Conservation of Energy

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

For any system in its entiretyas simple as a
swinging pendulum or as complex as an exploding
galaxythere is one quantity that does not
change energy. Energy may change form, but the
total energy stays the same.
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9.7 Conservation of Energy
When energy is transformed, it is conserved,
meaning that it will change form without losing
its original amount of energy.
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9.7 Conservation of Energy
When the woman leaps from the burning building,
the sum of her PE and KE remains constant at each
successive position all the way down to the
ground.
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9.7 Conservation of Energy
Elastic potential energy will become the kinetic
energy of the arrow when the bow does work on the
arrow.
As you draw back the arrow in a bow, you do work
stretching the bow. The bow then has potential
energy. When released, the arrow has kinetic
energy equal to this potential energy. It
delivers this energy to its target.
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9.7 Conservation of Energy
Everywhere along the path of the pendulum bob,
the sum of PE and KE is the same. Because of the
work done against friction, this energy will
eventually be transformed into heat.
Non-useful work can also be called non-useful
energy!
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9.7 Conservation of Energy

• Why does a tennis ball eventually stop bouncing?
• Eventually, all of the total mechanical energy is
  transformed into non-useful energy (heat, sound,
  movement of fibers)

50 J PE 50 J KE
New height less than before means less PE stored
35 J PE
20 J PE
35 J KE
20 J KE
Bounce!
Bounce!
(bounce and so on!)
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Slides showing transformation of KE and PE

• Source http//www.physicsclassroom.com/mmedia/ind
  ex.cfm

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Watch how KE and gravitational PE transform
Where is the KE at the maximum? Where is the PE
at the maximum? How is PE stored?
80
Watch the change in height vs. the change in
speed!
How does the change in height affect KE and PE?
81
What happens to KE and TME when the brakes are
applied? What work is being done?
82
Watch the transfer of KE and PE.
What happens to the PE when the skier moves down
the hill? What happens to the KE and TME when the
skier travels over the unpacked snow? What work
is done?
83
Same work, more force, less displacement (from
left to right)
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9.1 Work

• think!
• Suppose that you apply a 60-N horizontal force to
  a 32 kg package, which pushes it 4 meters across
  a mailroom floor. How much work do you do on the
  package?

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9.1 Work

• think!
• Suppose that you apply a 60-N horizontal force to
  a 32-kg package, which pushes it 4 meters across
  a mailroom floor. How much work do you do on the
  package?
• Answer
• W Fd 60 N 4 m 240 J

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9.7 Conservation of Energy
Total Mechanical Energy
Total Mechanical Energy
Same energy transformation applies
Non-mechanical Energy (dissipated)
10 J of PE does 8 J useful work on the arrow and
2 J of non-useful work on the molecules that
compose the bow and string and arrow. The arrow
has 8 J of KE as a result.
The 2 J of heat can be called non-useful work
(work that is not part of the objects total
mechanical energy).
Dissipated energy (DE) is amount of energy
transferred away from the total mechanical
energy. More DE means less KE, which reduces
TME, which means less speed!
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9.7 Conservation of Energy
Total Mechanical Energy
Total Mechanical Energy
Non-mechanical Energy (dissipated)
The 2 J of heat can be called non-useful work
(work that is not part of the objects total
mechanical energy).
Dissipated energy (DE) is amount of energy
transferred away from the total mechanical
energy. More DE means less KE, which reduces
TME, which means less speed!
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• Energy can change from one form to another
  without a net loss or gain.

LAW OF CONSERVATION OF ENERGY!!! (You will learn
to identify these transformations)