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Section 3: What Is Energy?
Key Ideas
What is the relationship between energy and work?
Why is potential energy called energy of position?
What factors does kinetic energy depend on?
What is nonmechanical energy?
Why It Matters
Some of the energy that reaches Earth from the sun is stored in plants. This energy is converted to work
by the animals that consume plants.
The world around us is full of energy. Energy exists in many forms. A lightning bolt has electrical energy.
A flashlight battery has chemical energy. A moving bicycle has mechanical energy. A rock sitting still on
top of a mountain also has mechanical energy, simply because it could move downhill! We harness various
forms of energy to power tools and machines, from flashlights to submarines.
Energy and Work
A moving object has energy associated with its motion. The mallet that the person in Figure 1 is
swinging has energy. When the mallet hits the lever, the mallet’s energy is transferred to the puck, the
puck rises to strike the bell, and the bell releases energy into the air as sound. Work has been done.
Whenever work is done, energy is transformed or is transferred from one system to another
system. In fact, one definition of energy is “the ability to do work.”
Figure 1 The moving mallet has energy and can do work on the puck. The transfer of energy causes the
puck to rise against gravity and ring the bell.
Energy is measured in joules. Although work is done only when an object experiences a change in its position or its
motion, energy can be present in an object or a system that is at rest. The energy in an object can be calculated whether
the object is in motion or at rest. The transfer of energy from one object or system to another, such as the transfer of
energy from the puck to the bell in Figure 1, can be measured by how much work is done on the receiving object.
Because energy is the ability to do work, measurements of energy and work are expressed in the same units—joules.
Potential Energy
When you stretch a rubber band, you do work. The energy used to stretch the rubber band is stored until
you release the rubber band. When you release the rubber band, it flies from your hand. But where is the
energy between the time you stretch the rubber band and the time you release the rubber band?
A stretched rubber band stores energy in a form called potential energy.
Potential energy (PE) is
sometimes called energy of position because it results from the relative positions of objects in
a system. Any object that is stretched or compressed to increase or decrease the distance between its
parts has potential energy that is called elastic potential energy. Stretched bungee cords and compressed
springs have elastic potential energy.
Imagine that you are at the top of the first hill of the roller coaster shown in Figure 2. Your position
above the ground gives you energy that could potentially do work as you move toward the ground. Any
system of two or more objects separated by a vertical distance has potential energy that results from the
gravitational attraction between the objects. This kind of stored energy is called gravitational potential
energy.
Figure 2 This roller coaster car has gravitational potential energy. This energy results from the
gravitational attraction between the car and Earth.
have?
What kind of energy does a stretched rubber band
Gravitational potential energy depends on both mass and height.
An apple at the top of a tree has more gravitational potential energy with respect to the Earth than an
apple of the same size on a lower branch does. But if two apples of different masses are at the same
height, the heavier apple has more gravitational potential energy than the lighter one does.
Because it is caused by the force of gravity, gravitational potential energy near Earth depends on both the
mass of the object and the height of the object relative to Earth’s surface.
Gravitational potential energy equation
grav. PE = mass × free-fall acceleration × height
PE = mgh
Notice that mg is the weight of the object in newtons, which is equal to the force on the object due to
gravity. So, like work, gravitational potential energy is calculated by multiplying force and distance.
Height is relative.
The value for h in the equation for gravitational potential energy is often measured from the ground. But
in some cases, a different height may be more important. For example, if an apple were about to fall into
a bird’s nest on a branch below the apple, the apple’s height above the nest would be used to calculate
the apple’s potential energy with respect to the nest.
Kinetic Energy
Once an object begins to move, it has the ability to do work. Consider an apple that falls from a tree. The
apple can do work when it hits the ground or lands on someone’s head. The energy that an object has
because it is moving is called kinetic energy.
The kinetic energy (KE) of an object depends on the object’s mass. A bowling ball can do more work than
a tabletennis ball if both balls are moving at the same speed. The kinetic energy of the object also
depends on the object’s rate of acceleration. An apple that is falling at 10 m/s can do more work than an
apple that is falling at 1 m/s. In fact, the kinetic energy of a moving object depends on the square of the
object’s speed.
Kinetic energy depends on both the mass and the speed of an object.
Kinetic energ equation
Figure 3 shows a graph of kinetic energy versus speed for a snowboarding student who has a mass of 50
kg. Notice that kinetic energy is expressed in joules. Because kinetic energy is calculated by using mass
and speed squared, kinetic energy is expressed in kilograms times meter squared per second squared (kg
• m2/s2), which is equivalent to joules.
Figure 3 A small increase in speed causes a large increase in kinetic energy. Kinetic energy varies as the
speed squared.
How would the kinetic energy of this snow boarder change if
he were wearing a heavy backpack?
Kinetic energy depends on speed more than it depends on mass.
You may have heard that car crashes are more dangerous at speeds above the speed limit than
at the speed limit. The kinetic energy equation provides a scientific reason. In the kinetic energy
equation, speed is squared, so a small change in speed causes a large change in kinetic energy.
Because a car has much more kinetic energy at high speeds, it can do much more work—and
thus much more damage—in a collision.
Atoms and molecules have kinetic energy.
Atoms and molecules are always moving. Therefore, these tiny particles have kinetic energy.
The motion of particles is related to temperature. The higher the kinetic energy of the atoms and
molecules in an object is, the higher the object’s temperature is.
Other Forms of Energy
An apple that is falling from a tree has both kinetic and potential energy. The sum of the kinetic
energy and the potential energy in a system is called mechanical energy. Mechanical energy
can also be thought of as the amount of work that something can do because of its kinetic and
potential energies.
An apple can give you energy when you eat it. What form of energy is that? In almost every
system, there are hidden forms of energy that are related to the arrangement of atoms that
make up the objects in the system.
Energy that lies at the level of the atom is sometimes called nonmechanical
energy. However, a close look at the different forms of energy in a system reveals that in most
cases, nonmechanical forms of energy are just special forms of either kinetic or potential energy.
Chemical reactions involve potential energy.
Chemical energy is a kind of potential energy. In a chemical reaction, bonds between atoms
break apart. When the atoms form new bonds, a different substance is formed. Both the
formation of bonds and the breaking of bonds involve changes in energy. The amount
of chemical energy in a substance depends in part on the relative positions of the atoms in the
substance.
Reactions that release energy decrease the potential energy in a substance. For example, when
a match is struck, as shown in Figure 4, the release of stored energy from the match head
produces light and a small explosion of hot gas.
Figure 4 When a match is struck, the chemical energy stored inside the head of the match is
released as light and heat.
What does chemical energy depend on?
Living things get energy from the sun.
Where does the lightning bug shown in Figure 5 get the energy
to glow? Where do you get the energy you need to live? The
energy comes from food. When you eat a meal, you eat plants,
animals, or both. Animals also eat plants, other animals, or
both. Plants and algae do not need to eat, because they get
their energy directly from sunlight.
Figure 5 A lightning bug produces light through an efficient
chemical reaction in its abdomen. Over 95% of the chemical
energy is converted to light.
What other plants and animals produce light?
Plants use photosynthesis to turn the energy in sunlight into chemical energy. This energy is
stored in sugars and other organic molecules that make up cells in living tissue. Thus, when you
eat a meal, you are really eating stored energy. When your body needs energy, some organic
molecules are broken down through respiration. Respiration releases the energy your body
needs in order to live and do work.
The sun gets energy from nuclear reactions.
The sun, shown in Figure 6, not only gives energy to living things but also keeps our whole
planet warm and bright. And the energy that reaches Earth from the sun is only a small portion
of the sun’s total energy output. How does the sun produce so much energy?
Figure 6 The nuclei of atoms contain enormous amounts of energy. The sun is fueled by nuclear
fusion reactions in its core.
The sun’s energy comes from nuclear fusion, a kind of reaction in which light atomic nuclei
combine to form a heavier nucleus. This nuclear energy is a kind of potential energy stored by
the forces holding subatomic particles together in the nuclei of atoms.
Nuclear power plants use a different process, called nuclear fission, to release nuclear energy. In
fission, a single large nucleus is split into two or more smaller nuclei. In both fusion and fission,
small quantities of mass are converted into large quantities of energy.
Energy can be stored in fields.
The lights and appliances in your home are powered by another form of energy, electrical
energy. Electrical energy results from the location of charged particles in an electric field. An
electric field is similar to a gravitational field. Certain places have high electric potential, while
others have low electric potential. When electrons move from an area of higher electric potential
to an area of lower electric potential, they gain energy. Moving electrons also create magnetic
fields, which can do work to power a motor. Electrons moving through the air between the
ground and a cloud cause the lightning shown in Figure 7.
Figure 7 Electrical energy is derived from the flow of charged particles, as in a bolt of lightning
or in a wire. We can harness electricity to power appliances in our homes.
Why It Matters: REAL WORLD
Energy Stored in Plants
The sun floods Earth with a large amount of energy in the form of electromagnetic radiation. Nature has a
unique way to store this energy from the sun. Green plants, algae, and some kinds of bacteria convert
solar energy into chemical potential energy through the process of photosynthesis. Photosynthesis is a set
of chemical reactions that use solar energy, carbon dioxide, and water to produce carbohydrates and
oxygen. Chemical reactions in the bodies of animals and humans on Earth convert carbohydrates into
work and thermal energy.
Light can carry energy across empty space.
Consider a bright summer day at a beach such as the one shown in Figure 8. Is it hotter where
light is shining directly on the sand or under the shade of the umbrella? You might guess,
correctly, that a seat in the direct sunlight is hotter. The reason is that light carries energy.
Figure 8 Electromagnetic waves carry energy from the sun to Earth.
Why is it cooler in the shade than in the sun?
Light energy travels from the sun to Earth across empty space in the form ofelectromagnetic
waves. Electromagnetic waves are made of electric and magnetic fields, so light energy is
another example of energy stored in a field. You will learn more about light and waves in
another chapter.