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15.2 – Energy Conservation
Conservation of Energy – Energy Conversion
Learning Objectives Section 15.1
 Describe conversion of energy from one form to another.
 State and apply the Law of Conservation of Energy.
 Analyze how energy is conserved in conversions
between kinetic and potential energy and solve
equation that equates initial energy to final energy.
 Describe the relationship between energy and mass and
calculate how much energy is equivalent to given mass.
Can energy be converted from one
form to another?
Yes!!!!!!!!!
The process of changing energy from one
form to another is energy conversion. The
striking of a match is a good example.
• Muscles use chemical energy to move the
match.
• Friction between the match and the matchbox
converts kinetic energy into thermal energy.
• Chemical energy is converted into thermal
energy and electromagnetic energy in the flame.
What is the Law of Conservation of
Energy?
 The law of conservation of energy states that energy
cannot be created or destroyed.
 When energy changes from one form to another, the
total energy remains unchanged, even though many
energy conversions may occur.
 In a closed system, the amount of energy present at the
beginning of a process is the same as the amount of
energy at the end.
More on Conservation of Energy
The work done by friction changes kinetic
energy into thermal energy.
• Friction within machinery reduces efficiency.
Friction is a major cause of energy
consumption in cars and factories.
• In many cases, most of a falling object’s
potential energy is converted into thermal
energy because of air resistance.
What energy conversion takes place
object falls towards Earth?
The gravitational potential energy of an object
is converted to the kinetic energy of motion as
the object falls.
One of the most common energy conversions is
between potential energy and kinetic energy.
• An avalanche brings tons of snow from the top of a
mountain to the valley floor.
• The elastic potential energy of a compressed spring is
converted into kinetic energy as the spring expands.
• Energy conversions can go from kinetic to potential
energy or from potential to kinetic energy
The Energy Conversion In Pendulums
A pendulum consists of a weight swinging
back and forth from a rope or string.
• At the highest point in its swing, the pendulum
has zero kinetic energy and maximum potential
energy.
• As the pendulum swings downward, potential
energy is converted to kinetic energy.
• At the bottom of the swing, the pendulum has
maximum kinetic energy and zero potential
energy.
.
Energy Conversion and the Pole Vault
In the pole vault, an athlete uses a flexible
pole to propel himself over a high bar.
Some of the pole-vaulter’s kinetic energy is
converted into elastic potential energy as
the pole bends. The pole springs back into
shape, propelling the pole-vaulter upward.
• As the pole-vaulter rises, his kinetic
energy decreases while he gains
gravitational potential energy.
• Once the highest point has been
reached, his gravitational potential
energy begins to convert back to
kinetic energy.
Energy Conversion Calculations
When friction is small enough to be ignored, and no
mechanical energy is added to a system, then the
system’s mechanical energy does not change.
Mechanical energy = KE + PE
The law of conservation of energy applies to any
mechanical process. If friction can be neglected, the
total mechanical energy remains constant.
Practice Problem Conservation of
Energy
At a construction site, a 1.50-kg brick is dropped from rest and hits
the ground at a speed of 26.0 m/s. Assuming air resistance can be
ignored, calculate the gravitational potential energy of the brick
before it was dropped.
Step1- Givens
Mass = 1.50kg
Vi = 0.0m / s
V f = 26.0m / s
PEg = ?
Step2 - formula
Before = After
KEi +PEi = KE f + PE f
Step3- Solve
Before = After
KEi +PEi = KE f + PE f
1
2 2
1
2 2
0
+PE
=
1.50kg·(26.0m
/
s
) +0
i
0 +PEi = 1.50kg·(26.0m / s ) + 0
2
2
PE i = 507kg·m 2 / s 2 = 507J
How are energy and mass related?
Einstein’s equation, E = mc2, says that energy
and mass are equivalent and can be
converted into each other.
Albert Einstein developed his special theory
of relativity in 1905. This theory included the
now-famous equation E = mc2.
• E is energy, m is mass, and c is the
speed of light.
• The speed of light is an extremely large
number, 3.0 × 108 meters per second.
• A tiny amount of matter can produce
an enormous amount of energy.
.
Example of Energy and Mass
Suppose 1 gram of matter were entirely converted into
energy.
E = mc2
= (10–3 kg) × (3 × 108 m/s) × (3 × 108 m/s)
= 9 × 1013 kg•m2/s2
= 9 × 1013 J
1 gram of TNT produces only 2931 joules of energy.
More on Energy and Mass
In nuclear fission and fusion
reactions, however, large
amounts of energy are
released by the destruction of
very small amounts of matter.
The law of conservation of
energy has been modified to
say that mass and energy
together are always
conserved.