Download Conservation of Energy

Table of Contents
Work and Energy
Section 1 • Work and Machines
Section 2 • Describing Energy
Section 3 • Conservation of Energy
Section
1
Work and Machines
What is work?
• To many people, the word work means something they
do to earn money.
• The word work also means exerting a force with your
muscles.
Section
1
Work and Machines
What is work?
• Someone might say they have done work when they
push as hard as they can against a wall that doesn't
move.
• However, in science the word work is used in a
different way.
Section
1
Work and Machines
What is work?
• Remember that a force is a push or a pull. Work
requires both force and motion.
• Work is force applied through a distance.
• If you push against the desk and nothing moves, then
you haven't done any work.
Section
1
Work and Machines
Doing work
• There are two conditions that have to be satisfied for
work to be done on an object.
• One is that the applied force must make the object
move, and the other is that the movement must be
in the same direction as the applied force.
Section
1
Work and Machines
Doing work
• For example, when you lift a stack of books, your arms
apply a force upward and the books move upward.
Because the force and distance are in the same
direction, your arms have done work on the books.
Section
1
Work and Machines
Force and Direction of Motion
• When you carry books while walking, you might
think that your arms are doing work.
• However, in this case, the force exerted by your
arms does no work on the books.
Section
1
Work and Machines
Force and Direction of Motion
• The force exerted by your arms on the books is
upward, but the books are moving horizontally.
• The force you exert is at right angles to the
direction the books are moving.
Section
1
Work and Machines
Calculating Work
• The amount of work done depends on the amount of
force exerted and the distance over which the force
is applied.
• When a force is exerted and an object moves in the
direction of the force, the amount of work done can be
calculated as follows.
Section
1
Work and Machines
Calculating Work
• In this equation, force is measured in newtons and
distance is measured in meters.
• Work, like energy, is measured in joules.
• One joule is about the amount of work required to
lift a baseball a vertical distance of 0.7 m.
Section
1
Work and Machines
When is work done?
• Suppose you give a book a push and it slides along a
table for a distance of 1 m before it comes to a stop.
• Even though the book moved 1 m, you do work on the
book only while your hand is in contact with it.
Section
Work and Machines
1
Machines
• A machine is a device that change the force or increase
the motion from work.
• Machines can be simple.
• Some, like knives,
scissors, and
doorknobs, are used
everyday to make
doing work easier.
Photodisc/PunchStock
Section
1
Work and Machines
Making Work Easier
• Some machines, such as bicycles, increase speed.
• Some machines, such as an axe, change the direction
of force.
• Some machines, such as a car jack, increase force.
Section
1
Work and Machines
Types of Simple Machines
• A simple machine is a machine that does work with
only one movement of the machine.
• There are six types of simple machines: lever, pulley,
wheel and axle, inclined plane, screw and wedge.
• The pulley and the wheel and axle are modified levers.
• The screw and the wedge are modified inclined planes.
Section
Work and Machines
1
Efficiency
• Efficiency is a measure of how much of the work put
into a machine is changed into useful output work by the
machine.
• Every machine is less than 100% efficient.
Section
1
Work and Machines
Calculating Efficiency
• To calculate the efficiency of a machine, the output work
is divided by the input work.
• Efficiency is usually expressed as a percentage by this
equation:
Section
1
Work and Machines
Increasing Efficiency
• Machines can be made more efficient by reducing
friction. This usually is done by adding a lubricant, such
as oil or grease, to surfaces that rub together.
• A lubricant fills in the gaps
between the surfaces, enabling
the surfaces to slide past each
other more easily.
Section
1
Work and Machines
Mechanical Advantage
• Two forces are involved when a machine is used to
do work.
• The force that is applied to the machine is called the
input force.
• Fin stands for the effort force.
• The force applied by the machine is called the output
force, symbolized by Fout.
Section
1
Work and Machines
Mechanical Advantage
• The ratio of the output force to the input force is the
mechanical advantage of a machine.
• The mechanical advantage of a machine can be
calculated from the following equation.
Section
1
Work and Machines
Mechanical Advantage
• Window blinds are a machine that changes the
direction of an input force.
• A downward pull on
the cord is changed
to an upward force
on the blinds.
Section
1
Work and Machines
Mechanical Advantage
• The input and output forces are equal, so the MA is 1.
Section
Section Check
1
Question 1
__________ is force applied through a distance.
A.
B.
C.
D.
Conversion
Energization
Power
Work
Section
1
Section Check
Answer
The answer is D. In order for work to be done, a force
must be applied through a distance.
Section
1
Section Check
Question 2
The amount of work done depends on what two
things?
Answer
The amount of work done depends on the amount of
force exerted and the distance over which the force is
applied.
Section
1
Section Check
Question 3
What do a knife, a doorknob, and a car jack have in
common?
Answer
These are all machines, because they are devices that
make doing work easier.
Section
1
Section Check
Question 4
What is the effect of increasing a machine’s
efficiency?
Answer
Increasing efficiency increases the ratio of output work
to input work.
Section
2
Describing Energy
What is energy?
• Wherever you are sitting as you read this, changes are
taking place—lightbulbs are heating the air around
them, the wind might be rustling leaves, or sunlight
might be glaring off a nearby window.
• Every change that occurs—large or small—involves
energy.
Section
2
Describing Energy
Change Requires Energy
• When something is able to change its environment or
itself, it has energy. Energy is the ability to cause
change.
• Anything that causes change must have energy.
• You use energy to arrange your hair to look the way you
want it to.
• You also use energy when you walk down the halls of
your school between classes or eat your lunch.
Section
2
Describing Energy
Work Transfer Energy
• Energy can also be described as the ability to do
work.
• Therefore, energy can be measured with the same units
as work.
• Energy, like work, can be measured in joules.
Section
Describing Energy
2
Systems
• It is useful to think of systems when describing energy.
• A system is anything that you can imagine a boundary
around.
• A system can be a single object, such as a baseball, or
a group of objects, such as the solar system.
Section
2
Describing Energy
Different Forms of Energy
• Energy has several different forms. Electrical, chemical,
radiant, and thermal are examples.
• Is the chemical energy from food the same as the energy
that comes from the Sun or the energy from gasoline?
• Radiant energy from the Sun travels a vast distance
through space to Earth, warming the planet and
providing energy that enables green plants to grow.
Section
2
Describing Energy
An Energy Analogy
• If you have $100, you could store it in a variety of
forms—cash in your wallet, a bank account, travelers’
checks, or gold or silver coins.
• You could convert that money into different forms.
Section
2
Describing Energy
An Energy Analogy
• You could deposit your cash into a bank account or
trade the cash for gold.
• Regardless of its form, money is money.
• The same is true for energy.
• Energy from the Sun that warms you and energy from
the food that you eat are only different forms of the
same thing.
Section
2
Describing Energy
Kinetic Energy
• An object in motion does have energy.
• Kinetic energy is the energy a moving object has
because of its motion.
• The kinetic energy of a moving object depends on the
object’s mass and its speed.
Section
2
Describing Energy
Potential Energy
• Even motionless objects can
have energy. This energy is
stored energy.
• A hanging apple in a tree has
stored energy.
Pixtal/age fotostock
Section
2
Describing Energy
Potential Energy
• Stored energy due to the
interactions between objects is
potential energy.
• If the apple stays in the tree,
the energy will remain stored.
Pixtal/age fotostock
Section
2
Describing Energy
Potential Energy
• If the apple falls, that stored
energy is converted to kinetic
energy.
Pixtal/age fotostock
Section
2
Describing Energy
Elastic Potential Energy
• If you stretch a rubber band and let it go, it sails across
the room.
• As it flies through the air, it has kinetic energy due to its
motion.
• Where did this kinetic energy come from?
Section
2
Describing Energy
Elastic Potential Energy
• The stretched rubber band had energy stored as elastic
potential energy.
• Elastic potential energy is energy stored by
something that can stretch or compress, such as a
rubber band or spring.
Section
2
Describing Energy
Chemical Potential Energy
• Gasoline, food, and other substances have chemical
potential energy.
• Energy stored due to chemical bonds is chemical
potential energy.
Section
2
Describing Energy
Chemical Potential Energy
• Energy is stored due to the bonds that hold the atoms
together and is released when the gas is burned.
• In this chemical reaction, chemical potential energy is
released.
Section
2
Describing Energy
Gravitational Potential Energy
• Together, an object near Earth and Earth itself have
gravitational potential energy.
• Gravitational potential energy (GPE) is energy due to
gravitational forces between objects.
Section
2
Describing Energy
Gravitational Potential Energy
• Gravitational potential energy can be calculated from the
following equation.
• Near Earth’s surface, gravity is 9.8 N/kg.
• Like all forms of energy, gravitational potential energy
can be measured in joules.
Section
2
Describing Energy
Changing GPE
• According to the equation for gravitational potential
energy, the GPE of an Earth system can be increased
by increasing the object’s height.
• Gravitational potential energy also increases if the
mass of the object increases.
Section
Section Check
2
Question 1
Energy is the ability to cause __________.
A.
B.
C.
D.
change
heat
motion
work
Section
2
Section Check
Answer
The answer is A. Energy is the ability to cause
change and has several different forms.
Section
2
Section Check
Question 2
What are four different forms of energy?
Answer
Answers will vary. Different forms of energy include
electrical, chemical, radiant, thermal, nuclear, mechanical,
potential, and gravitational.
Section
Section Check
2
Question 3
The kinetic energy due to an object’s motion depends
only on __________.
A.
B.
C.
D.
the object’s mass and speed
the object’s mass
the object’s speed
the acceleration of the object
Section
2
Section Check
Answer
The answer is A. Kinetic energy depends on both the
mass and speed of the moving object.
Section
3
Conservation of Energy
The Law of Conservation of Energy
• Energy can change from one form to another, but the
total amount of energy never changes.
• Even when energy
changes form, energy is
never destroyed.
Section
3
Conservation of Energy
The Law of Conservation of Energy
• This principle is recognized as a law of nature.
• The law of conservation of energy states that energy
cannot be created or destroyed.
Section
3
Conservation of Energy
Conserving Resources
• You might have heard about energy conservation or
been asked to conserve energy.
• These ideas are related to reducing the demand for
electricity and gasoline, which lowers the consumption
of energy resources such as coal and fuel oil.
Section
3
Conservation of Energy
Conserving Resources
• The law of conservation of energy, on the other hand, is
a universal principle that describes what happens to
energy as it is transferred from one object to another or
as it is transformed.
Section
3
Conservation of Energy
Energy Transformations
• You are more likely to think of energy as race cars roar
past or as your body uses energy from food to help it
move, or as the Sun warms your skin on a summer day.
• These situations involve energy changing from one
form to another form.
Section
3
Conservation of Energy
Mechanical Energy Transformations
• Mechanical energy is the sum of the kinetic energy and
potential energy of the objects in a system.
• Often, the mechanical energy of a system remains
constant or nearly constant.
• In these cases, energy is only converted between
different forms of mechanical energy.
Section
3
Conservation of Energy
Falling Objects
• An apple-Earth system on a
tree has gravitational potential
energy due to the gravitational
force between apple and Earth.
• The instant the apple comes
loose from the tree, it
accelerates due to gravity.
Pixtal/age fotostock
Section
3
Conservation of Energy
Falling Objects
• As the apple falls, it loses
height so the gravitational
potential energy decreases.
• This potential energy is
transformed into kinetic energy
as the speed of the apple
increases.
Pixtal/age fotostock
Section
3
Conservation of Energy
Falling Objects
• If the gravitational potential energy is being converted
into completely into the kinetic energy of the apple
falling, then the mechanical energy of the system does
not change as the apple falls.
• The potential energy that the apple loses is gained back
as kinetic energy.
• The form of energy changes, but the total amount of
energy remains the same.
Section
3
Conservation of Energy
Energy Transformations in Projectile Motion
• Energy transformations also occur during projectile
motion when an object moves in a curved path.
Section
3
Conservation of Energy
Energy Transformations in Projectile Motion
• However, the mechanical energy of the ball-Earth
system remains constant as it rises and falls.
Section
3
Conservation of Energy
Energy Transformations in a Swing
• When you ride on a swing part of the fun is the feeling
of almost falling as you drop from the highest point to
the lowest point of the swing’s path.
Section
3
Conservation of Energy
Energy Transformations in a Swing
• The ride starts with a push that gets you moving, giving
you kinetic energy.
• As the swing rises, you lose speed but gain height.
• In energy terms, kinetic energy changes to gravitational
potential energy.
Section
3
Conservation of Energy
Energy Transformations in a Swing
• At the top of your path, potential energy is at its
greatest.
• Then, as the swing accelerates downward, potential
energy changes to kinetic energy.
Section
3
Conservation of Energy
Is energy always conserved?
• While coasting along a flat road on a bicycle, you know
that you will eventually stop if you don’t pedal.
• If energy is conserved, why wouldn’t your kinetic energy
stay constant so that you would coast forever?
Section
3
Conservation of Energy
The Effect of Friction
• You know from
experience that if you
don’t continue to pump a
swing or get a push from
somebody else, your arcs
will become lower and
you eventually will stop
swinging.
Section
3
Conservation of Energy
The Effect of Friction
• In other words, the mechanical (kinetic and potential)
energy of the swing decreases, as if the energy were
being destroyed. Is this a violation of the law of
conservation of energy?
Section
3
Conservation of Energy
The Effect of Friction
• With every movement, the swing’s ropes or chains
rub on their hooks and air pushes on the rider.
• Friction and air
resistance cause some
of the mechanical
energy of the swing to
change to thermal
energy.
Section
3
Conservation of Energy
The Effect of Friction
• With every pass of the swing, the temperature of
the hooks and the air increases a little, so the
mechanical energy of the swing is not destroyed.
• Rather, it is transformed into thermal energy.
Section
3
Conservation of Energy
Transforming Electrical Energy
• Lightbulbs transform electrical energy into light so you
can see.
• The warmth you feel
around the bulb is
evidence that some
of that electrical
energy is transformed
into thermal energy.
Section
3
Conservation of Energy
Transforming Chemical Energy
• Fuel stores chemical potential energy.
• The engine transforms the chemical potential energy
of gasoline molecules into the kinetic energy of a
moving car or bus.
Section
3
Conservation of Energy
Transforming Chemical Energy
• Several energy conversions occur in
this process.
• In a car, a spark plug fires, initiating
the conversion of chemical potential
energy into thermal energy.
Section
3
Conservation of Energy
Transforming Chemical Energy
• As the hot gases expand,
thermal energy is converted into
kinetic energy.
Section
3
Conservation of Energy
Transforming Chemical Energy
• Some energy transformations are less obvious because
they do not result in visible motion, sound, heat or light.
• Every green plant you see converts the radiant energy
from the Sun into the energy stored due to the chemical
bonds in the plant.
Section
3
Conservation of Energy
Power—how fast energy changes
• The rate at which energy is converted is the object’s
power.
• Power is measured in watts with 1 watt equaling 1 joule
per second.
Section
3
Conservation of Energy
The Human Body—Balancing the
Energy Equation
• What forms of energy can you find in the human body?
• With your right hand, reach up and feel your left
shoulder.
• With that simple action, potential energy from your body
was converted to the kinetic energy of your moving arm.
Section
3
Conservation of Energy
The Human Body—Balancing the
Energy Equation
• Some of your body’s the chemical potential energy is
used to maintain a nearly constant internal temperature.
• A portion of this energy also is converted to the
excess thermal energy that your body gives off to
its surroundings.
Section
3
Conservation of Energy
Energy Conversions in Your Body
• Fat and other chemical compounds store energy
for your body.
• This chemical potential energy is used to fuel the
processes that keep you alive, such as making
your heart beat and digesting the food you eat.
Section
3
Conservation of Energy
Energy Conversions in Your Body
• You also use this energy to make your body move.
Section
3
Conservation of Energy
Food Energy
• The food Calorie (C) is a unit used by nutritionists to
measure how much energy you get from various
foods—1 C is equivalent to about 4,000 J.
• Every gram of fat a person consumes can supply about
10 C of energy.
• Carbohydrates and proteins each supply about 5 C of
energy per gram.
Section
3
Conservation of Energy
Conservation of Energy
• Energy can be converted between its many forms,
including mechanical energy, thermal energy, electrical
energy, and chemical energy.
• The law of conservation of energy states that energy
never can be created or destroyed. The total amount
of energy in the universe is constant.
Section
Section Check
3
Question 1
________ energy is the sum of the kinetic energy and
potential energy of the objects in a system.
A.
B.
C.
D.
Kinetic
Potential
Thermal
Mechanical
Section
3
Section Check
Answer
The answer is D. Mechanical energy is the sum of the
kinetic energy and potential energy of the objects in a
system.
Section
3
Section Check
Question 2
State the law of conservation of energy.
Answer
The law of conservation of energy states that energy
cannot be created or destroyed.
Section
Section Check
3
Question 3
Friction converts __________ energy into ___________
energy.
A.
B.
C.
D.
electrical, thermal
mechanical, thermal
thermal, electrical
thermal, mechanical
Section
3
Section Check
Answer
The answer is B. Friction converts mechanical energy into
thermal energy.
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Chapter Resources
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Video Clips and Animations
Chapter Summary
Chapter Review Questions
Standardized Test Practice
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Using Simple Machines
Photodisc/PunchStock
THUMBNAILS
Image Bank
Lubricant
THUMBNAILS
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Input and Output Force
THUMBNAILS
Image Bank
Potential Energy
Pixtal/age fotostock
THUMBNAILS
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Chemical Potential Energy
THUMBNAILS
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Transforming Chemical Potential Energy to
Thermal Energy
THUMBNAILS
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Transforming Thermal Energy to Kinetic Energy
THUMBNAILS
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Kinetic and Gravitational Potential Energy
THUMBNAILS
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Energy Transformation
THUMBNAILS
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Law of Conservation of Energy
THUMBNAILS
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The Effect of Friction
THUMBNAILS
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Slowing Down
THUMBNAILS
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Using Calories
THUMBNAILS
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Transforming Electrical Energy
THUMBNAILS
Video Clips and Animations
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Video Clips and Animations
Reviewing Main Ideas
Work
• Work is the transfer of energy when a force makes an
object move.
• Work is done only when force produces motion in the
direction of the force.
• Power is the amount of work, or the amount of energy
transferred, in a certain amount of time.
Reviewing Main Ideas
Using Machines
• A machine makes work easier by changing the size of
the force applied, by increasing the distance an object
is moved, or by changing the direction of the applied
force.
• The number of times a machine multiplies the force
applied to it is the mechanical advantage of the
machine. The actual mechanical advantage is always
less than ideal mechanical advantage.
Reviewing Main Ideas
Using Machines
• The efficiency of a machine equals the output work
divided by the input work.
• Friction always causes the output work to be less than
the input work, so no real machine can be 100 percent
efficient.
Reviewing Main Ideas
Simple Machines
• A simple machine is a machine that can do work with a
single movement.
• A simple machine can increase an applied force,
change its direction, or both.
Reviewing Main Ideas
Simple Machines
• A lever is a bar that is free to pivot about a fixed point
called a fulcrum. A pulley is a grooved wheel with a rope
running along the groove. A wheel and axle consists of
two different-sized wheels that rotate together. An
inclined plane is a sloping surface used to raise objects.
The screw and wedge are special types of inclined
planes.
• A combination of two or more simple machines is called
a compound machine.
Reviewing Main Ideas
The Nature of Energy
• Energy is the ability to cause change.
• Energy can have different forms, including kinetic,
potential, and thermal energy.
• Moving objects have kinetic energy that depends on
the object’s mass and velocity, and can be calculated
from this equation:
Reviewing Main Ideas
The Nature of Energy
• Potential energy is stored energy. An object can have
gravitational potential energy that depends on its mass
and its height, and is given by this equation:
GPE = mgh
Reviewing Main Ideas
Conservation of Energy
• Energy can change from one form to another.
Devices you use every day transform one form of
energy into other forms that are more useful.
• Falling, swinging, and projectile motion all involve
transformations between kinetic energy and
gravitational potential energy.
Reviewing Main Ideas
Conservation of Energy
• Friction converts mechanical energy into thermal
energy, causing the mechanical energy of a system to
decrease.
• Mass is converted into energy in nuclear fission and
fusion reactions. Fusion and fission occur in the nuclei
of certain atoms, and release tremendous amounts of
energy.
Chapter Review
Question 1
How does adding a lubricant affect the friction, output
force, and efficiency of a machine?
Answer
Lubricants fill spaces between surfaces and reduce
friction. Therefore, the output force and efficiency of
the machine are greater.
Chapter Review
Question 2
What is the difference between kinetic and
potential energy?
Answer
Kinetic energy is the energy a moving object has because
of its motion; potential energy is stored energy due to
position.
Chapter Review
Question 3
A lightbulb converts electrical energy into
_______and _______.
Answer
The filament of a lightbulb converts the electrical energy
supplied to the bulb into thermal energy and radiant
energy.
Chapter Review
Question 4
What is the difference between energy and power?
Answer
Power is the rate at which energy is converted from one
form to another.
Chapter Review
Question 5
The SI unit of energy is the __________.
A.
B.
C.
D.
calorie
joule
Newton
watt
Chapter Review
Answer
The answer is B. One joule is 1 kg·m2/s2.
Chapter Review
Question 6
What type of energy is stored due to the bond between
atoms?
A.
B.
C.
D.
chemical kinetic energy
chemical potential energy
elastic potential energy
elastic kinetic energy
Chapter Review
Answer
The answer is B. Chemical potential energy is energy
stored due to chemical bonds.
Standardized Test Practice
Question 1
You move a 130-kg dresser at an acceleration of 0.5 m/s2
over a distance of 5 m. How much work do you do on the
dresser?
A.
B.
C.
D.
65 J
260 J
325 J
1300 J
Standardized Test Practice
Answer
The answer is C. Calculate the force applied by
multiplying mass by acceleration. Calculate work done by
multiplying force by distance.
Standardized Test Practice
Question 2
How long will it take a jogger to convert 1,000 J of
energy if her power is 125 W?
A.
B.
C.
D.
8s
125 s
250 s
1250 s
Standardized Test Practice
Answer
The answer is A. Use the equation P =E/t and solve for t.
Standardized Test Practice
Question 3
Calculate the mechanical advantage of a crowbar if the
input force is 200 N and the output force is 1800 N
A.
B.
C.
D.
2000
0.1
0.9
9
Standardized Test Practice
Answer
The answer is D. Mechanical advantage is equal to the
output force divided by the input force.
Standardized Test Practice
Question 4
What is the kinetic energy of a 7.5-kg salmon swimming at
0.67 m/s?
A.
B.
C.
D.
1.7 J
2.5 J
3.4 J
5.0 J
Standardized Test Practice
Answer
The answer is A. Kinetic energy is equal to one-half the
mass multiplied by the square of the velocity.
Standardized Test Practice
Question 5
A runner has a mass of 60 kg and a kinetic energy of 750
J. What is the runner’s speed?
A.
B.
C.
D.
4 m/s
5 m/s
9 m/s
10 m/s
Standardized Test Practice
Answer
The answer is B. Use the formula KE = ½ mv2 and solve
for v.
Standardized Test Practice
Question 6
Find the approximate kinetic energy of a ball with a mass
of 0.05 kg moving at 35 m/s.
A.
B.
C.
D.
1225 J
61 J
31 J
1J
Standardized Test Practice
Answer
The answer is C. Use the formula KE = ½ mv2.
Standardized Test Practice
Question 7
Use the table to determine
approximate how long a
person would need to run
in order to use the same
number of calories as a
person uses walking in 1 h.
Standardized Test Practice
A.
B.
C.
D.
5 min
10 min
15 min
20 min
Standardized Test Practice
Answer
The answer is C. A runner burns calories about four
times as fast as a walker.
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