Transcript 15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation
As a meteor traveled
through the atmosphere
in October 1992, some
of its kinetic energy was
converted into light and
heat. Upon impact,
much of the meteor's
remaining kinetic
energy went into
smashing the rear of
this car in Peekskill,
New York.
15.2 Energy Conversion and Conservation
Energy Conversion
Can energy be converted from one form into
another?
Energy can be converted from one form to
another.
15.2 Energy Conversion and Conservation
Energy Conversion
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.
15.2 Energy Conversion and Conservation
Energy Conversion
Energy is converted
from one form to
another as this match
is lit.
15.2 Energy Conversion and Conservation
Conservation of Energy
What is the law of conservation of energy?
The law of conservation of energy states
that energy cannot be created or destroyed.
15.2 Energy Conversion and Conservation
Conservation of Energy
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.
15.2 Energy Conversion and Conservation
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.
15.2 Energy Conversion and Conservation
Conservation of Energy
Although speed skaters slide quickly over
smooth ice, they are still slowed down by
friction with the air and the surface of the ice.
15.2 Energy Conversion and Conservation
Energy Conversions
What energy conversion takes place as an
object falls toward Earth?
The gravitational potential energy of an
object is converted to the kinetic energy of
motion as the object falls.
15.2 Energy Conversion and Conservation
Energy Conversions
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.
15.2 Energy Conversion and Conservation
Energy Conversions
Some gulls use energy conversion to obtain
food by dropping oysters onto rocks. Kinetic
energy causes the shell to break on collision
with the rock.
15.2 Energy Conversion and Conservation
Energy Conversions
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.
15.2 Energy Conversion and Conservation
Energy Conversions
Pendulum clocks use
pendulums to maintain
accurate time.
The time it takes for a
pendulum to swing back and
forth once is precisely related
to its length.
15.2 Energy Conversion and Conservation
Energy Conversions
Energy Conversion and the Pole Vault
In the pole vault, an athlete uses
a flexible pole to propel himself
over a high bar.
15.2 Energy Conversion and Conservation
Energy Conversions
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.
15.2 Energy Conversion and Conservation
Energy Conversions
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
15.2 Energy Conversion and Conservation
Energy Conversions
The law of conservation of energy applies to
any mechanical process. If friction can be
neglected, the total mechanical energy
remains constant.
15.2 Energy Conversion and Conservation
Energy Conversions
Conservation of Mechanical 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.
15.2 Energy Conversion and Conservation
Energy Conversions
Read and Understand
What information are you given?
What unknown are you trying to calculate?
15.2 Energy Conversion and Conservation
Energy Conversions
Read and Understand
What information are you given?
What unknown are you trying to calculate?
15.2 Energy Conversion and Conservation
Energy Conversions
Plan and Solve
What equations or formulas contain the given
quantities and the unknown?
15.2 Energy Conversion and Conservation
Energy Conversions
Plan and Solve
What equations or formulas contain the given
quantities and the unknown?
Because the brick falls without air resistance, the
conservation of mechanical energy equation can
be used.
15.2 Energy Conversion and Conservation
Energy Conversions
Plan and Solve
You will also need to use the formula for kinetic energy (KE).
Note that the KE at the beginning is zero because the brick
has not yet begun to fall. Also, when the brick hits the ground,
its potential energy is zero. Substitute these values into the
conservation of energy formula.
15.2 Energy Conversion and Conservation
Energy Conversions
Plan and Solve
Substitute the formula for KE.
Substitute the known values and calculate the PE.
15.2 Energy Conversion and Conservation
Energy Conversions
Plan and Solve
Substitute the formula for KE.
Substitute the known values and calculate the PE.
15.2 Energy Conversion and Conservation
Energy Conversions
Look Back and Check
Is your answer reasonable?
15.2 Energy Conversion and Conservation
Energy Conversions
Look Back and Check
Is your answer reasonable?
Check the answer by finding the initial height
of the brick, using PE = 507 J = mgh.
Substituting in m and g gives h = 34.5 m.
This is a reasonable height for an object in
free fall to reach a speed of 26.0 m/s.
15.2 Energy Conversion and Conservation
Energy Conversions
1. A 10-kg rock is dropped and hits the ground
below at a speed of 60 m/s. Calculate the
gravitational potential energy of the rock before
it was dropped. You can ignore the effects of
friction.
Answer:
15.2 Energy Conversion and Conservation
Energy Conversions
1. A 10-kg rock is dropped and hits the ground
below at a speed of 60 m/s. Calculate the
gravitational potential energy of the rock before
it was dropped. You can ignore the effects of
friction.
Answer:
(PE)beginning = (KE)end = ½mv2
=(0.50)(10 kg)(60 m/s)2 = 18,000 J
15.2 Energy Conversion and Conservation
Energy Conversions
2. A diver with a mass of 70.0 kg stands motionless at
the top of a 3.0-m-high diving platform. Calculate his
potential energy relative to the water surface while
standing on the platform, and his speed when he enters
the pool. (Hint: Assume the diver’s initial vertical speed
after diving is zero.)
Answer:
15.2 Energy Conversion and Conservation
Energy Conversions
2. A diver with a mass of 70.0 kg stands motionless at
the top of a 3.0-m-high diving platform. Calculate his
potential energy relative to the water surface while
standing on the platform, and his speed when he enters
the pool. (Hint: Assume the diver’s initial vertical speed
after diving is zero.)
Answer:
15.2 Energy Conversion and Conservation
Energy Conversions
3. A pendulum with a 1.0-kg weight is set in motion from a
position 0.04 m above the lowest point on the path of the
weight. What is the kinetic energy of the pendulum at the
lowest point? (Hint: Assume there is no friction.)
Answer:
15.2 Energy Conversion and Conservation
Energy Conversions
3. A pendulum with a 1.0-kg weight is set in motion from a
position 0.04 m above the lowest point on the path of the
weight. What is the kinetic energy of the pendulum at the
lowest point? (Hint: Assume there is no friction.)
Answer:
(PE)beginning = mgh
= (1.0 kg)(9.8 m/s2)(0.04 m) = 0.4 J;
at the beginning, KE = 0, and at the lowest point, PE = 0;
therefore (PE)beginning = (KE)end = 0.4 J
15.2 Energy Conversion and Conservation
Energy and Mass
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.
15.2 Energy Conversion and Conservation
Energy and Mass
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.
15.2 Energy Conversion and Conservation
Energy and Mass
Albert Einstein made
important contributions
to many areas of
physics.
His theory of special
relativity showed that
energy and mass are
equivalent.
15.2 Energy Conversion and Conservation
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.
15.2 Energy Conversion and Conservation
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.
15.2 Energy Conversion and Conservation
Assessment Questions
1. What energy conversion occurs as a result of
friction?
a.
b.
c.
d.
chemical energy to thermal energy
kinetic energy to potential energy
kinetic energy to thermal energy
potential energy to thermal energy
15.2 Energy Conversion and Conservation
Assessment Questions
1. What energy conversion occurs as a result of
friction?
a.
b.
c.
d.
chemical energy to thermal energy
kinetic energy to potential energy
kinetic energy to thermal energy
potential energy to thermal energy
ANS: C
15.2 Energy Conversion and Conservation
Assessment Questions
2. At what point in a pendulum’s swing does it have
maximum kinetic energy?
a.
b.
c.
d.
the highest point of the swing
the lowest point of the swing
halfway between the lowest and highest point
same at all positions of the swing
15.2 Energy Conversion and Conservation
Assessment Questions
2. At what point in a pendulum’s swing does it have
maximum kinetic energy?
a.
b.
c.
d.
the highest point of the swing
the lowest point of the swing
halfway between the lowest and highest point
same at all positions of the swing
ANS: C
15.2 Energy Conversion and Conservation
Assessment Questions
3. Based on Einstein’s equation for the equivalence
of energy and mass, how much energy is
produced by the conversion of 1 kilogram of mass
to energy?
a.
b.
c.
d.
3x103 J
3x105 J
9x105 J
9x1013 J
15.2 Energy Conversion and Conservation
Assessment Questions
3. Based on Einstein’s equation for the equivalence
of energy and mass, how much energy is
produced by the conversion of 1 kilogram of mass
to energy?
a.
b.
c.
d.
3x103 J
3x105 J
9x105 J
9x1013 J
ANS: D
15.2 Energy Conversion and Conservation
Assessment Questions
1. According to the law of conservation of mass,
energy can be converted from one from to another
but not created or destroyed.
True
False
15.2 Energy Conversion and Conservation
Assessment Questions
1. According to the law of conservation of mass,
energy can be converted from one from to another
but not created or destroyed.
True
False
ANS:
F, law of conservation of energy