1 Rdg_7.1_What_Is_Energy

Chapter 7 ENERGY

7.1 What Is Energy?

Unlike matter, pure energy cannot be smelled, tasted, touched, seen, or heard. However, energy energy - a quantity that describes
does appear in many forms, such as motion and heat. Energy can also travel in different ways,
such as in light and as electricity. Without energy, nothing could ever change. In fact, the workings the ability of an object to change or
of the entire universe (including all of our technology) depend on energy flowing and changing cause changes.
back and forth from one form to another.
joule (J) - a unit of energy. One joule
Defining energy
is enough energy to push with a force
What is energy? Energy describes the ability of things to change themselves or to cause of 1 newton for a distance of 1 meter.
change in other things. What types of changes are we talking about? Some
examples are changes in temperature, speed, position, pressure, or any other Figure 7.1: Pushing a 1-kilogram
physical variable. Energy can also cause changes in materials, such as when
burning wood changes into ashes and smoke. object with a force of 1 newton for a
distance of 1 meter uses 1 joule of
What has energy? The list below describes objects that have energy. Read through this list and energy.
notice how many different forms of energy exist. We will talk more about
these different forms in this chapter. Units Related to the Joule
1 joule = 1 newton-meter
Measuring energy • A gust of wind has energy because it can move objects in its path. 1 newton = 1 kg-m/s2
• A piece of wood burning in a fireplace has energy because it can produce therefore...
1 joule = 1 kg-m2/s2
heat and light.
• You have energy because you can change the motion of your body.
• Batteries have energy; they can be used in a radio to make sound.
• Gasoline has energy; it can be burned in an engine to move a car.
• A ball at the top of a hill has energy because it can roll down the hill and

move objects in its path.
A joule (J) is the unit of measurement for energy. One joule is the energy
needed to push with a force of 1 newton for a distance of 1 meter
(Figure 7.1). So, 1 joule is equivalent to 1 newton multiplied by 1 meter (or
1 newton-meter). If you push a toy car forward with a force of 1 newton over
a distance of 1 meter, you have applied 1 joule of energy to the car. One joule
is a pretty small amount of energy. An ordinary 100-watt electric light bulb
uses 100 joules of energy every second!

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Some forms of energy

Understanding One way to understand energy is to think of it as nature’s money. Energy can mechanical energy - a form of
energy be spent and saved in a number of different ways. It takes energy to “buy”
changes like going faster, moving higher, or getting hotter. These three energy that is related to motion or
changes use energy. The opposite changes, such as slowing down, falling, or position. Potential and kinetic energy
cooling off, release energy. Just like a checkbook, nature keeps perfect track are examples.
of energy. What you “spend” diminishes what you have left. You can only
“buy” as much change as you have energy to “pay for.” chemical energy - a form of

Mechanical energy Mechanical energy is the energy possessed by an object due to its motion or potential energy that is stored in
its position. This means potential energy and kinetic energy are both forms of molecules.
mechanical energy.
STUDY SKILLS
Chemical energy Chemical energy is a form of energy
stored in molecules. Batteries are really Keeping Track of Energy
storage devices for chemical energy. For In this section, you will learn about
example, the chemical energy in a battery different forms of energy. Keep track
changes to electrical energy when you of these in a table. List the name of
connect wires and a light bulb to the battery. each form of energy and write down
Your body also uses chemical energy when any information you learn about it.
it converts food into energy so that you can
walk or think. A car and many other types
of machines use chemical energy when they
burn fuel to operate.

Electrical energy Electrical energy comes from electric charge, which is one of the fundamental
properties of all matter. You will learn more about electricity and electric
charge in Unit 7. The electrical energy we use in our homes is transformed
from other forms of energy, such as the chemical energy released by burning
oil and gas, or the mechanical energy released by falling water in a
hydroelectric dam or power plant.

Pressure energy Pressure in gases and liquids is also a form of energy. An inflated bicycle tire
has more energy than a flat tire. An inflated tire can hold up a bicycle (with
you on it) against the force of gravity while a flat tire cannot.

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Chapter 7 ENERGY

More forms of energy

Elastic energy Elastic energy is energy that is stored or released when an object changes nuclear energy - a form of energy
shape (or deforms). For example you use energy to stretch a rubber band.
Some of the energy from your muscles is stored as elastic energy in the that is stored in the nuclei of atoms.
stretched (changed) shape of the rubber band. The energy is released again
when the rubber band changes back to its original (unstretched) shape. radiant energy - a form of energy
Objects that are commonly used to store and release elastic energy include
rubber bands, springs, and archery bows (Figure 7.2). that is represented by the
electromagnetic spectrum.

Nuclear energy and Every second, about 5 million tons of mass is converted to energy through
radiant energy nuclear reactions in the core of the Sun. In the Sun, nuclear energy is
transformed to heat that eventually escapes the Sun as radiant energy.
Nuclear energy is a form of energy stored in the nuclei of atoms (particles
of matter). You will read more about nuclear energy and nuclear reactions in
Chapter 18. Radiant energy is energy that is carried by electromagnetic
waves. Light is one form of radiant energy, and so are radio waves that carry
music through the air.

The electromagnetic Light and radio waves are a traveling form of pure energy. In fact, they are
spectrum only two of a whole family of energy waves called the electromagnetic
spectrum. The electromagnetic spectrum includes infrared radiation (heat),
visible light (what we see), and ultraviolet light. In other words, light energy
and heat energy are included in the electromagnetic spectrum. You will
recognize other components of the spectrum as well. You have listened to
radio waves, may have cooked with microwaves, and maybe you have had
an image made of a part of your body with X-rays.

Figure 7.2: A stretched bowstring on

a bent bow has elastic energy, so it is
able to create change in itself and in the
arrow.

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The Sun and gravity

The Sun and energy Both living creatures and human technology derive virtually all of their
energy from the Sun. Without the Sun’s energy, Earth would be a cold, icy
place with a temperature of –273 degrees Celsius. The Sun’s energy not only
warms the planet, it also drives the entire food chain (Figure 7.3). Plants store
the energy as carbohydrates, like sugar. Animals eat the plants to get energy.
Other animals eat those animals for their energy. It all starts with the Sun.

Life on Mars and A very important question in science today
other planets is whether there is life on other planets such
as Mars. Mars is farther from the Sun than
Earth. For this reason, Mars receives less Figure 7.3: The flow of energy from
energy from the Sun than does Earth. In
fact, the average temperature on Mars is the Sun supports all living things on
well below the freezing point of water. Can Earth.
life exist on Mars? Recent research suggests
that it may be possible. Scientists have
found bacteria in the Antarctic ice living at
a temperature colder than the average
temperature of Mars.

Gravity and energy A falling rock gains speed as it falls. Energy must be supplied to increase The planet Venus is closer to the Sun
speed. The falling water that turns a hydroelectric turbine must also have than Earth. Should this make Venus
energy, otherwise no electrical energy could be produced. Where does this warmer or colder than Earth?
energy come from? Research your answer to see what
scientists think Venus is like on its
surface.

The answer has to do with Earth’s gravity. If an object, or any matter, is lifted
against gravity, energy is stored. This stored energy is transformed into
energy of motion, such as the object falling back down. Many forms of human
technology, including roller coasters, swings, water wheels, hydroelectric
power plants, and even a kind of medieval catapult called a trebuchet, rely on
gravity.

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Chapter 7 ENERGY

Energy and work

What work means In physics, the word work has a very specific meaning. Work is the transfer of
in physics energy that results from applying a force over a distance. Work is a product of
the force applied times the distance traveled (work = force × distance). For
example, if you push a block with a force of 1 newton for a distance of
1 meter, you do 1 joule of work. Both work and energy are measured in the
same units (joules) because work is a form of energy.

Work and Doing work always means transferring energy. The energy may be
potential energy transferred to the object to which force is applied, or it may go somewhere
else. For example, you can increase the energy of a rubber band by exerting a
force that stretches it. The work you do stretching the rubber band is stored
as elastic potential energy by the rubber band. The rubber band can then use
that stored energy to do work on a paper airplane, giving it energy
(Figure 7.4).

Work is done When thinking about work, you should always be clear about which force is
on objects doing the work on which object. Work is done on objects. If you lift a block
1 meter with a force of 1 newton, you have done 1 joule of work on the
block.

Energy is needed An object that has energy is able to do work; without energy, it is impossible
to do work to do work. In fact, energy can sometimes be thought of as stored work. As
the block you lifted earlier falls, it has energy that can be used to do work. If
the block hits a ball, it will do work on the ball and change the ball’s motion.
Some of the block’s energy is transferred to the ball during the collision
(left). You will learn more about the concept of work in the next chapter.

Figure 7.4: You can do work to

increase an object’s energy. Then that
energy can do work on another object,
giving it energy.

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Potential energy

What is potential Potential energy is energy due to position. The word potential means that potential energy - energy due to
energy? something is capable of becoming active. Systems or objects with potential position.
energy are able to exert forces (exchange energy) as they change to other
arrangements. For example, a stretched spring has potential energy. If
released, the spring will use this energy to move itself (and anything attached
to it) back to its original length.

Gravitational A block suspended above a table has potential energy. If released, the force of
potential energy gravity moves the block down to a position of lower energy. The term
gravitational potential energy describes the energy of an elevated object. The
term is often shortened to just potential energy because the most common
type of potential energy in physics problems is gravitational. Unless
otherwise stated, you can assume potential energy means gravitational
potential energy.

How to calculate How much potential energy does a raised block have? The block’s potential
potential energy energy is exactly the amount of work it can do as it goes down. Work is force
multiplied by distance. The force is the weight (mg) of the block in newtons.
The distance the block can move down is its height (h) in meters. Multiplying
the weight by the distance gives you the block’s potential energy at any given
height (Figure 7.5).

POTENTIAL ENERGY

Mass (kg)

E = mghPotential energy Height (m)
(J)
p
Acceleration
Figure 7.5: The potential energy of
due to gravity (9.8 m/s2)
the block is equal to the product of its
mass, the strength of gravity, and the
height from which the block can fall.

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Kinetic energy

Kinetic energy is Objects that are moving also have the ability to cause change. Energy of kinetic energy - energy of
energy of motion motion is called kinetic energy. A moving billiard ball has kinetic energy motion.
because it can hit another ball and change its motion. Kinetic energy can
easily be converted into potential energy. The kinetic energy of a basketball
tossed upward converts into potential energy as the height increases.

Kinetic energy The amount of kinetic energy an object has equals the amount of work the
can do work object can do by exerting force as it stops. Consider a moving skateboard and
rider (Figure 7.6). Suppose it takes a force of 500 newtons applied over a
distance of 10 meters to slow the skateboard down to a stop (500 N × 10 m =
5,000 joules). The kinetic energy of the skateboard and rider is 5,000 joules
since that is the amount of work it takes to stop the skateboard.

Kinetic energy If you had started with twice the mass—say, two skateboarders—you would
depends on mass have to do twice as much work to stop them both. Kinetic energy increases
with mass. If the skateboard and rider are moving faster, it also takes more
and speed work to bring them to a stop. This means kinetic energy also increases with
speed. Kinetic energy is related to both an object’s speed and its mass.

The formula for The kinetic energy of a moving object is equal to one half its mass multiplied
kinetic energy
by the square of its speed. This formula comes from a combination of

relationships, including Newton’s second law, the distance equation for
acceleration (d = 1/2at2), and the calculation of energy as the product of force
and distance.

Figure 7.6: The amount of kinetic

energy the skateboard has is equal to the
amount of work that must be done to
stop the skateboard.

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Solving Problems: Potential and Kinetic Energy Kinetic Energy and Speed

A 2 kilogram rock is at the edge of a cliff 20 meters above a lake. The rock Kinetic energy increases as the square
becomes loose and falls toward the water below. Calculate its potential and of the speed. This means that if you go
kinetic energy when it is at the top and when it is halfway down. Its speed is twice as fast, your energy increases by
14 m/s at the halfway point. four times (22 = 4). If your speed is
three times as fast, your energy is nine
1. Looking for: You are asked for the potential and kinetic energy at two locations. times bigger (32 = 9). A car moving at a
speed of 100 km/h (62 mph) has four
2. Given: You are given the mass in kilograms, the height at each location in meters, times the kinetic energy it had when
and the speed halfway down in m/s. You can assume the initial speed is going 50 km/h (31 mph). At a speed of
3. Relationships: 0 m/s because the rock starts from rest. 150 km/h (93 mph), it has nine times as
4. Solution: much energy as it did at 50 km/h.
Ep = mgh and Ek = 1 mv2 m = 2 kg, g = 9.8 N/kg, and h = 20 m The stopping distance of a car is
Ep = (2 kg)(9.8 N/kg)(20 m) = 392 J proportional to its kinetic energy. A car
2 going twice as fast has four times the
kinetic energy and needs four times the
Potential energy at the top: stopping distance. This is why driving
at high speeds is so dangerous.
Potential energy halfway down: m = 2 kg, g = 9.8 N/kg, and h = 10 m
Ep = (2 kg)(9.8 N/kg)(10 m) = 196 J a. 117.6 J
b. 50 J
Kinetic energy at the top: m = 2 kg and v = 0 m/s
Ek = (1/2)(2 kg)(02) = 0 J

Kinetic energy halfway down: m = 2 kg and v = 14 m/s
Ek = (1/2)(2 kg)(14 m/s)2 = 196 J

Your turn...

a. Calculate the potential energy of a 4-kilogram cat crouched 3 meters off
the ground.

b. Calculate the kinetic energy of a 4-kilogram cat running at 5 m/s.

7.1 WHAT IS ENERGY? 171