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Chapter 15
Energy!!!
In an avalanche, a mass of
loose snow, soil, or rock
suddenly gives way and slides
down the side of a mountain.
The avalanche releases a great
amount of energy.
Energy vs Work
 Energy is the ability to do work.
 Work is a transfer of energy.
Energy & Work
Work and energy are closely related.
• Energy is known by the changes it causes.
• Work is done when a force moves an object through a
distance. Energy is transferred by a force moving an
object through a distance.
• Both work and energy are typically measured in joules
(J).
Energy Forms
Energy has different forms.
A. The sun gives off energy in the form of heat and light.
B. Plants convert sunlight into food.
C. People convert food energy into muscle movement.
Kinetic Energy
 The energy of motion is called kinetic energy.
 Doubling the mass in the formula doubles the kinetic
energy.
 Doubling the speed quadruples the kinetic energy.
Calculating KE
 A 0.10-kilogram bird is flying at a constant speed of 8.0
m/s. What is the bird’s kinetic energy?
Calculating KE
 A 50.0-kilogram cheetah has a kinetic energy of 18,000
J. How fast is the cheetah running? (Hint: Rearrange
the equation to solve for v.)
Potential Energy
 Potential energy is energy that is stored as a result of
position or shape.
PE
 When this musician pulls
the string of her cello to
one side, the string is
stretched and gains
potential energy.
PE
The musician adds energy to the cello string by
plucking it.
• The energy stored in the stretched string is potential
energy.
• The stored energy is converted into kinetic energy when
the string is released and it vibrates.
PE
Gravitational Potential Energy
Potential energy that depends upon an object’s height is
called gravitational potential energy.
This type of potential energy increases when an object is
raised to a higher level.
PE
This diver has gravitational
potential energy as she stands
at the end of a diving board.
She gained the potential energy
by doing work—by climbing up
the steps to the diving board.
PE Equation
 The unit for mass is kilograms.
 The unit for height is meters.
 Acceleration due to gravity, g, has a value in SI units of
9.8 m/s2 on Earth.
 The unit for gravitational potential energy is joules.
Solving for PE
What is the potential energy relative to the water surface
of a diver at the top of a 10.0-meter-high diving platform.
Suppose she has a mass of 50.0 kilograms.
PE = mgh
= (50.0 kg)(9.8 m/s2)(10.0 m)
= 4900 kg•m2/s2 = 4900 J
Elastic PE
Elastic Potential Energy
The potential energy of an object that is stretched or
compressed is known as elastic potential energy.
Something that is elastic springs back to its original
shape after it is stretched or compressed.
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.
Law of Conservation of Energy
 The law of conservation of energy states that energy
cannot be created or destroyed.
 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.
Friction
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.
KE to PE or PE to KE
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.
Energy Conversion Example
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.
Equation
 The law of conservation of energy applies to any
mechanical process. If friction can be neglected, the
total mechanical energy remains constant.
Solve
 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
 http://www.youtube.com/watch?v=qZ4FFWvZtyo
Forms of Energy
 The major forms of energy
are mechanical energy,
thermal energy, chemical
energy, electrical energy,
electromagnetic energy,
and nuclear energy.
Forms of Energy
All energy can be considered to be one of three
forms:
• kinetic energy,
• potential energy, and
• energy in fields such as those produced by
electromagnetic waves.
Each of these forms of energy can be converted into
other forms of energy.
Forms of Energy
Mechanical Energy
The energy associated with the motion and position of
everyday objects is mechanical energy.
Mechanical energy is the sum of an object’s potential
energy and kinetic energy.
Forms
of
Energy
Thermal Energy
The total potential and kinetic energy
of all the microscopic particles in an
object make up its thermal energy.
When an object’s atoms move faster,
its thermal energy increases, and the
object becomes warmer.
Forms of Energy
Chemical Energy
Chemical energy is the energy
stored in chemical bonds.
When bonds are broken, the
released energy can do work.
All chemical compounds,
including fuels such as coal and
gasoline, store energy.
Forms of Energy
Electrical Energy
Electrical energy is the energy associated with electric
charges.
Electric charges can exert forces that do work.
Forms of Energy
Electromagnetic Energy
Electromagnetic energy is a form of energy that travels
through space in the form of waves.
 Visible light and X-rays are examples of electromagnetic
energy.
Forms of Energy
Nuclear Energy
The nucleus of an atom is held together by strong
and weak nuclear forces, which can store an
enormous amount of potential energy.
• The energy stored in atomic nuclei is known as nuclear
energy.
• Nuclear fission releases energy by splitting nuclei apart.
• Nuclear fusion releases energy when less massive nuclei
combine to form a more massive nucleus.
EINSTEIN!!
 Einstein’s equation, E = mc2,
says that energy and mass are
equivalent and can be
converted into each other.
Einstein
Albert Einstein developed his special theory of
relativity in 1905. This theory included the nowfamous 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.
Nonrenewable Energy
 Nonrenewable energy resources exist in limited
quantities and, once used, cannot be replaced except
over the course of millions of years.
 Nonrenewable energy resources include oil, natural gas,
coal, and uranium.
 Oil, natural gas, and coal are known as fossil fuels.
Renewable Energy
 Renewable energy resources are resources that can
be replaced in a relatively short period of time.
 Renewable energy resources include hydroelectric, solar,
geothermal, wind, biomass, and, possibly in the future,
nuclear fusion.
Ultimate Source of OUR Energy
Most renewable energy resources originate either
directly or indirectly from the sun.
• The sun and Earth are constantly releasing large
amounts of energy.
• This energy could be used for generating electric power,
heating buildings, or other purposes.
Renewable Energy
Hydroelectric Energy
Energy obtained from flowing
water is known as hydroelectric
energy.
Renewable Energy
Solar Energy
 Sunlight converted into usable energy is called solar
energy.
Renewable Energy
Geothermal Energy
 Geothermal energy is thermal energy beneath
Earth’s surface.
Renewable Energy
Other Renewable Resources
The chemical energy stored in living things is called
biomass energy.
A hydrogen fuel cell generates electricity by reacting
hydrogen with oxygen.