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
The process of changing energy from one form to
another is _______________________________.
•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
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 ___________, 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
The work done by friction changes kinetic energy into
________________________.
• Friction reduces________and 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.
• 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
Gravitational potential energy of an object is converted
to kinetic energy of motion as the object falls.
One of the most common energy conversions is
between___________energy and__________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
A __________is a weight swinging back and
forth from a rope or string.
• At the highest point, a pendulum has 0
kinetic energy and max potential energy.
• As the pendulum swings down, potential
energy is converted to kinetic energy.
• At the bottom of the swing, the pendulum
has maximum kinetic energy and 0
potential energy.
• The time it takes for a pendulum to swing
back and forth once is precisely related to
its length.
• Pendulum clocks use pendulums to
maintain accurate time.
15.2 Energy Conversion and Conservation
In the pole vault, an athlete uses a flexible pole to
propel over a high bar.
• Some of the pole-vaulter’s kinetic energy is
converted into_________potential energy as the pole
bends. The pole springs back into shape, propelling
the pole-vaulter upward.
• As the pole-vaulter rises, kinetic
energy decreases while he gains
gravitational potential energy.
• Once the highest point has been
reached, gravitational potential
energy begins to convert back to
kinetic energy.
15.2 Energy Conversion and Conservation
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
__________________process.
•If friction can be neglected, the total mechanical
energy remains __________________.
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
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.
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.)
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.)
15.2 Energy Conversion and Conservation
Albert Einstein developed his special theory of
relativity in 1905. This theory included the now-famous
equation________________.
• E is energy, m is mass, and c is
the speed of ___________.
• The speed of light is extremely
large: 3.0 × 108 m/s.
• Energy is released as matter is
destroyed and matter can be
created from energy: energy and
mass are equivalent.
• A tiny amount of matter can
produce an__________amount of
energy.
15.2 Energy Conversion and Conservation
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.
•In nuclear fission and fusion reactions, 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_______are always conserved.