Energy is inherent in all forms of matter. But unlike matter, energy is not a
substance and does not occupy space. Energy is defined as the capacity to perform
work. Two types of energy are recognized--potential and kinetic.
a. Potential Energy. Potential energy is the energy of position. It is the energy
inherent in a body because of position. Thus, a car parked on a hill has potential
energy. Water retained by a dam located upstream above a power plant also has
potential energy. They represent stored energy waiting to be released under the right
b. Kinetic Energy. Kinetic energy is the energy inherent in a moving body.
Thus, when the brakes of a car are released and it begins to move, the potential energy
becomes kinetic energy. When the dammed-up water falls to the power plant below,
the potential energy in the water becomes kinetic energy. As the potential energy
decreases in each of these instances, the kinetic energy increases, until finally the total
kinetic energy at point of impact (when the car hits the bottom of the hill and keeps
rolling, and when water from the dam begins to drive the turbines) equals the initial
c. Kinetic-Potential Energy Relationship. A swinging pendulum (figure 1-1)
also illustrates the kinetic-potential energy relationship. At the extreme upward end of
each swing (A and C), the bob comes momentarily to rest, and all the energy then is
potential. However, on its swing downward, the pendulum increases in kinetic energy
(a) and all energy at the bottom of the swing (B) is kinetic. As it swings from the bottom
upward toward the opposite end, the pendulum loses kinetic energy and gains potential
energy (b) until its contained energy is all potential energy again (at C).
Figure 1-1. Pendulum used to illustrate kinetic and potential energy.