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Chapter 10
Energy
Topics:
•
•
Important forms of energy
•
•
Definition of work
•
•
The law of conservation of energy
How energy can be transformed and
transferred
Concepts of kinetic, potential, and thermal
energy
Elastic collisions
Sample question:
When flexible poles became available for pole vaulting, athletes
were able to clear much higher bars. How can we explain this using
energy concepts?
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Slide 10-1
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Slide 10-3
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Slide 10-4
Forms of Energy
Mechanical Energy
Ug
K
Thermal
Energy
Us
Other forms include
E th
Echem
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Enuclear
Slide 10-12
The Basic Energy Model
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Slide 10-13
Energy Transformations
Kinetic energy K = energy of motion
Potential energy U = energy of position
Thermal energy Eth = energy associated with
temperature
System energy E = K + U + Eth + Echem + ...
Energy can be transformed within the
system without loss.
Energy is a property of a system.
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Slide 10-14
Some Energy Transformations
Echem  Ug
K  Eth
Echem  Ug
Us  K  U g
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Slide 10-15
Energy Transfers
These change the energy of the system.
Interactions with the environment.
Work is the mechanical transfer of energy to
or from a system via pushes and pulls.
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Slide 10-20
Energy Transfers: Work
W K
W Eth
W Us
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Slide 10-21
The Law of Conservation of Energy
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Slide 10-23
The Basic Equation
Kf  Uf  Eth  Ki  Ui + Esys
A few things to note:
• Esys can be positive (Energy in) or negative (Energy out)
• We are, for now, ignoring heat.
• Thermal energy is…special. When energy changes to
thermal energy, this change is irreversible.
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Slide 10-24
Energy Model
Types of Energy
Kinetic Energy KE = 1/2 mv2
Potential Energy:
Gravitational Potential Energy
Spring Potential Energy
PEg = mgy
Pes = 1/2 kL2
Conservation of Energy (Closed System)
Before
After
KEi  PEgi  PEsi  Esys  KE f  PEgf  PEsf  Eth
Visualizations:
• Energy Bar Charts
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Checking Understanding
A skier is moving down a slope at a constant speed. What energy
transformation is taking place?
A. EK => Eg
B. Eg => Eth
C. Es => Eg
D. Eg => EK
E. EK => Eth
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Slide 10-12
Answer
A skier is moving down a slope at a constant speed. What energy
transformation is taking place?
B. Ug  Eth
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Slide 10-13
Demonstration: Smash the Professor - Part 2
Energy Bar Charts for a swinging pendulum
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Slide 10-7
Examples of Energy Bar Charts
& solving Energy Problems
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Slide 10-7
Energy Bar Charts and Energy Transformations
For the following questions, start by drawing energy bar graphs
and identifying energy transformations. Then answer the question.
1. A block slides down a frictionless ramp of height h. It
reaches velocity v at the bottom. To reach a velocity of 2v,
the block would need to slide down a ramp of height
A. 1.41 h
h
B. 2 h
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C. 3 h
D. 4 h
E. 6
Slide 10-23
Energy Bar Charts and Energy Transformations
For the following questions, start by drawing energy bar graphs
and identifying energy transformations. Then answer the question.
2. A block is shot up a frictionless 40 degree slope with initial
veloctiy v. It reaches height h before sliding back down. The
same block is shot with the same velocity up a frictionless 20
degree slope. On this slope, the block reaches height
A. 2 h
B. h
C. 1/2 h
D. Greater than h, but I connot predict an exact value.
E. Less than h, but I can't predict an exact value
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Slide 10-23
Energy Bar Charts and Energy Transformations
For the following questions, start by drawing energy bar graphs
and identifying energy transformations. Then answer the question.
3. Two balls, one twice as heavy as the other, are dropped from
the roof of a building. Just before hitting the ground, the
heavier ball has _________ the kinetic energy of the lighter
ball.
A. one-half
B. the same amount as
C. twice
D. four times
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Slide 10-23
Conceptual Example Problem
A car sits at rest at the top of a hill. A small push sends it rolling
down a hill. After its height has dropped by 5.0 m, it is moving at a
good clip. Write down the equation for conservation of energy,
noting the choice of system, the initial and final states, and what
energy transformation has taken place.
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Slide 10-25
Example: Roller Coaster
Demonstration: which ball reaches the end of the track first
A. The one in front
B. The one in back
C. Neither, they both reach the end of the track at the same
time
Using Conservation of Energy to find the speed of a roller
coaster
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Slide 10-23