Energy

Energy is the ability to do work. Energy can be derived from physical or

chemical sources. For example, you use physical energy when you lift a book
up off of a table. An example of chemical energy is the electricity produced
from chemicals in a battery. The combustion in an automobile engine is
another example of chemical energy. The two main categories of energy are
potential and kinetic energy.

Potential and Kinetic Energy

To understand how energy is used in everyday life, you need a basic
understanding of potential and kinetic energy. Potential energy is the stored
energy that can be used to perform work. Nutrients that animals consume
contain stored chemical energy. The opposite of stored energy is energy of
motion. Kinetic energy is the energy of motion. Muscles move using kinetic
energy generated from the stored potential energy of nutrients. When sitting
on top of a slide, you have potential energy to slide downward. Using your
muscles, you use kinetic energy to glide down to the bottom of the slide.
 In the diagram, the child on a swing displays both potential and kinetic
energy in action. Potential energy is at its highest level when the child has
reached the top of the swinging motion. Kinetic energy increases as the child
begins to swing back toward the center of the swing set. Kinetic energy is at
its highest level at the bottom of this curve, while potential energy is at its
lowest value there.
This conversion of energy from potential to kinetic or kinetic to potential
is known as energy transformation. Energy transformation is the process of
changing from one form of energy to another. The food that you eat contains
chemical energy. This chemical energy in foods is then transformed into
kinetic energy, which gives you the power to move.

Forms of Energy
There are many forms of energy that can be either potential or kinetic, such as
chemical energy, gravitational energy, mechanical energy, radiant energy,
thermal energy, sound energy, and electrical energy. Two types of potential
energy are chemical energy and gravitational energy.
• Chemical energy is potential energy stored in the bonds of molecules.
This energy is found in food, batteries, and combustible natural fuels
such as coal and gas.
• Gravitational energy is the energy an object has because of its
position in relation to Earth. Gravitational energy is dependent on mass
and height. A book located on the top shelf of a bookcase has more
gravitational energy than a book located on the bottom shelf. In other
words, the higher an object is away from the center of Earth, the greater
the gravitational energy.
Chemical and gravitational energy are summarized in the table.

Kinetic energy is energy of motion. Any object that is in motion uses a

form of kinetic energy. There are many types of kinetic energy.
• The mechanical energy of an object is the sum of its energy in motion
and stored energy. An example of mechanical energy in use is a wind
turbine. The kinetic energy of the wind is combined with the potential
energy of the turbine to produce mechanical energy.
• Radiant energy is the energy of electromagnetic waves that travel
through a medium such as air, water, or solid matter. Some examples of
radiant energy include radio waves, X-rays, gamma rays, and light.
• Thermal energy is the vibration of atoms and molecules within a
material. As the material’s temperature increases, the intensity of
molecular vibration causes collisions that release thermal energy. When
thermal energy is transferred to or from an object, it is called heat.
 When combustion engines ignite fuel, chemical potential energy is
converted to thermal energy.
• Sound energy is energy produced by vibrating sound waves moving
through a medium. An example of sound energy is the plucking of a
guitar string sending vibrating waves into the air around it. Sound
energy requires a force to cause the object to vibrate.
• Electrical energy is the energy produced by the movement of electrons.
Storm clouds release electrical energy in the form of lightning, which is
similar to the static electricity created when you shuffle your feet across
a heavy carpet while wearing socks.
The many forms of kinetic energy are summarized in the table below.

Conservation of Energy
The energy that exists in the universe cannot be created or destroyed,
although its form may change. Potential energy converts to kinetic energy and
back again through cycles. This constant amount of energy is described by the
law of conservation of energy. The law of conservation of energy states that
the total amount of energy in the universe is constant.
 Recall that gravitational potential energy (GPE) is the energy possessed
by an object due to its mass or its position. For example, a boulder that
begins to roll down from the top of a hill contains its maximum potential
energy at the top of the hill. The higher that hilltop, the more potential energy
there is within the boulder. This can be shown by using the equation for GPE.
GPE = m × h × g
where m = mass (kg), h = height (m), g = gravitational constant
The gravitational constant is 9.8 meters per second squared (m/s2).
Energy is measured in joules (J). The GPE is affected by the object’s vertical
position on Earth—the higher the object, the greater the GPE.
Thus, a boulder that weighs 5,000 kg that is resting on a hill 100 m in
height will have a GPE of approximately 4,900,000, or 4.9 × 106 J.
GPE = 5,000 kg × 100 m × 9.8 m/s2 = 4.9 × 106 J
As the boulder rolls down the hill, its kinetic energy increases and its
potential energy decreases.
Recall that kinetic energy is the energy created by moving objects. The
kinetic energy of an object depends upon the object’s mass and velocity. The
following equation is used for calculating kinetic energy:

KE = 0.5 × m × v2
where m = mass (kg), v = velocity (m/s)
If the 5,000 kg boulder is rolling at a velocity of 5 m/s, its kinetic energy
at that moment is 62,500 J.
KE = 0.5 × 5,000 kg × (5 m/s)2 = 62,500 J
The conservation of energy states that the total amount of energy is
constant. Therefore, the total energy of an object is the sum of its potential
energy and kinetic energy. Recall that mechanical energy is the sum of an
object’s potential and kinetic energy. In this case, the total mechanical energy
of the boulder is equal to the sum of its GPE plus its kinetic energy.
 At the boulder’s highest point on the hill, the total amount of energy is
solely present as potential energy. As the boulder rolls down the hill, some
of the potential energy is converted to kinetic energy. The height of the
boulder from Earth is decreasing, which decreases the GPE. At the same
time, the velocity of the boulder as it rolls increases, which increases its
kinetic energy. When the boulder lands at the bottom of the hill, there is no
more GPE and the kinetic energy is at its maximum. The total mechanical
energy of the boulder is conserved as the boulder rolls down the hill.
We calculated the maximum GPE of the boulder at the top of the hill to be
4.9 × 106 J. Since the total amount of energy is constant and is equal to the
sum of potential and kinetic energy, the maximum kinetic energy of the
boulder as it crashes at the bottom of the hill must be equal to 4.9 × 106 J.
You know the mass of the boulder is unchanged at 5,000 kg. Knowing this,
you can determine the velocity of the boulder at the bottom of the hill. The
kinetic energy equation can be used to determine the velocity of the boulder
at the bottom of the hill by incorporating our known maximum kinetic energy
of 4.9 × 106 J.

The boulder must reach the velocity of about 44.27 m/s as it crashes at
the bottom of the hill, ensuring that the total energy is conserved.

EXERCISE 5

Energy
Directions: Choose the best answer to each question.
1. Which statement is true about energy?
A. Kinetic energy is a form of stored energy.
B. The potential energy of an object increases as the object moves.
C. Thermal energy is released by charged particles.
D. Potential energy is found in the nutrients we eat.
2. A crane lifts a 100-kg weight 10 m off the ground. The weight is
released and falls to the ground. At the instant the weight is released,
what is its GPE?
A. 0J
B. 98 J
C. 980 J
D. 9,800 J
3. A baseball is thrown by the pitcher at a velocity of 50 m/s. The
resulting kinetic energy from this action is equal to 187.5 J. What is the
approximate weight of the baseball in kilograms?
A. 0.015 kg
B. 0.150 kg
C. 1.500 kg
D. 6.7 kg