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Chapter 12
Section 3 What is Energy?
Objectives
• Explain the relationship between energy and work.
• Define potential energy and kinetic energy.
• Calculate kinetic energy and gravitational potential
energy.
• Distinguish between mechanical and nonmechanical energy.
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Chapter 12
Section 3 What is Energy?
Bellringer
In the chapter on matter, you learned that energy is
conserved. Instead of being created or destroyed, it is
just changed from one form to another. The energy of
the sunlight that reaches Earth is the ultimate source of
most of the energy around us. Look at the illustration
below, and identify the types of energy involved.
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Chapter 12
Section 3 What is Energy?
Bellringer, continued
1. How did energy from sunlight provide the energy the
girl needed to swing the bat? (Hint: What do you need
to have energy?)
2. When the girl hits the ball, she exerts a force on it.
Does she do work on the ball in the scientific sense of
the term? Explain why.
3. After the girl hits the ball, the ball moves very fast and
has energy. When the ball hits the fielder’s glove, it
stops moving. Given that energy can never be
destroyed but merely changes form, what happened to
the energy the ball once had? (Hint: If you are the
fielder, what do you hear and feel as you catch the
ball?)
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Chapter 12
Section 3 What is Energy?
Energy and Work
• Energy is the ability to do work.
• When you do work on an object, you transfer
energy to that object.
• Whenever work is done, energy is transformed or
transferred to another system.
• Energy is measured in joules.
• Because energy is a measure of the ability to do
work, energy and work are expressed in the same
units.
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Chapter 12
Section 3 What is Energy?
Potential Energy
• The energy that an object has because of the
position, shape, or condition of the object is called
potential energy.
• Potential energy is stored energy.
• Elastic potential energy is the energy stored in any
type of stretched or compressed elastic material,
such as a spring or a rubber band.
• Gravitational potential energy is the energy stored
in the gravitational field which exists between any
two or more objects.
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Chapter 12
Section 3 What is Energy?
Potential Energy
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Chapter 12
Section 3 What is Energy?
Potential Energy, continued
• Gravitational potential energy depends on both mass
and height.
• Gravitational Potential Energy Equation
grav. PE = mass  free-fall acceleration  height
PE = mgh
• The height can be relative.
• The height used in the above equation is usually
measured from the ground.
• However, it can be a relative height between two
points, such as between two branches in a tree.
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Chapter 12
Section 3 What is Energy?
Math Skills
Gravitational Potential Energy A 65 kg rock climber
ascends a cliff. What is the climber’s gravitational
potential energy at a point 35 m above the base of
the cliff?
1. List the given and unknown values.
Given:
mass, m = 65 kg
height, h = 35 m
free-fall acceleration, g = 9.8 m/s2
Unknown: gravitational potential energy, PE = ? J
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Chapter 12
Section 3 What is Energy?
Math Skills, continued
2. Write the equation for gravitational potential
energy.
PE = mgh
3. Insert the known values into the equation, and
solve.
PE = (65 kg)(9.8 m/s2)(35 m)
PE = 2.2  104 kg•m2/s2
PE = 2.2  104 J
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Chapter 12
Section 3 What is Energy?
Kinetic Energy
• The energy of a moving object due to the object’s
motion is called kinetic energy.
• Kinetic energy depends on mass and speed.
• Kinetic Energy Equation
1
kinetic energy   mass  speed squared
2
1
KE  mv 2
2
• Kinetic energy depends on speed more than mass.
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Chapter 12
Section 3 What is Energy?
Kinetic Energy Graph
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Chapter 12
Section 3 What is Energy?
Kinetic Energy
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Chapter 12
Section 3 What is Energy?
Math Skills
Kinetic Energy What is the kinetic energy of a 44 kg
cheetah running at 31 m/s?
1. List the given and unknown values.
Given:
mass, m = 44 kg
speed, v = 31 m/s
Unknown: kinetic energy, KE = ? J
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Chapter 12
Section 3 What is Energy?
Math Skills, continued
2. Write the equation for kinetic energy.
1
kinetic energy   mass  speed squared
2
1
KE  mv 2
2
3. Insert the known values into the equation, and
solve.
1
KE  (44 kg)(31 m/s)2
2
KE  2.1 10 4 kggm2 /s2
KE  2.1 10 4 J
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Chapter 12
Section 3 What is Energy?
Other Forms of Energy
• The amount of work an object can do because of the
object’s kinetic and potential energies is called
mechanical energy.
• Mechanical energy is the sum of the potential energy
and the kinetic energy in a system.
• In addition to mechanical energy, most systems
contain nonmechanical energy.
• Nonmechanical energy does not usually affect
systems on a large scale.
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Chapter 12
Section 3 What is Energy?
Other Forms of Energy, continued
• Atoms and molecules have kinetic energy.
• The kinetic energy of particles is related to heat
and temperature.
• Chemical reactions involve potential energy.
• The amount of chemical energy associated with a
substance depends in part on the relative
positions of the atoms it contains.
• Living things get energy from the sun.
• Plants use photosynthesis to turn the energy in
sunlight into chemical energy.
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Chapter 12
Section 3 What is Energy?
Other Forms of Energy, continued
• The sun gets energy from nuclear reactions.
• The sun is fueled by nuclear fusion reactions in its
core.
• Electricity is a form of energy.
• Electrical energy is derived from the flow of
charged particles, as in a bolt of lightning or in a
wire.
• Light can carry energy across empty space.
• Light energy travels from the sun to Earth across
empty space in the form of electromagnetic
waves.
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