Potential Energy
An object can store energy as the result of its position. For example, the heavy ball of a
demolition machine is storing energy when it is held at an elevated position. This stored
energy of position is referred to as potential energy. Similarly, a drawn bow is able to store
energy as the result of its position. When assuming its
usual position
(i.e., when not drawn),
there is no energy stored in the bow. Yet when its position is altered from its usual
equilibrium position, the bow is able to store energy by virtue of its position. This stored
energy of position is referred to as potential energy. Potential energy is the stored energy
of position possessed by an object.
Gravitational Potential Energy
The two examples above illustrate the two forms of potential energy to be discussed -
gravitational potential energy and elastic potential
energy. Gravitational potential energy is the energy stored in an
object as the result of its vertical position or height. The energy
is stored as the result of the gravitational attraction of the Earth
for the object. The gravitational potential energy of the massive ball of a demolition machine
is dependent on two variables - the mass of the ball and the height to which it is raised.
There is a direct relation between gravitational potential energy and the mass of an object.
More massive objects have greater gravitational potential energy. There is also a direct
relation between gravitational potential energy and the height of an object. The higher that
an object is elevated, the greater the gravitational potential energy. These relationships are
expressed by the following equation:
PEgrav = mass • g • height
PEgrav = m *• g • h
In the above equation, m represents the mass of the object, h represents the height of the
object and g represents the gravitational field strength (9.8 N/kg on Earth) - sometimes
referred to as the acceleration of gravity.
To determine the gravitational potential energy of an object,
a
zero height position
must first be arbitrarily assigned.
Typically, the ground is considered to be a position of zero
height. But this is merely an arbitrarily assigned position that
most people agree upon. Since many of our labs are done on
tabletops, it is often customary to assign the tabletop to be the
zero height position. Again this is merely arbitrary. If the
tabletop is the zero position, then the potential energy of an
object is based upon its height relative to the tabletop. For example, a pendulum bob
swinging to and from above the tabletop has a potential energy that can be measured based
on its height above the tabletop. By measuring the mass of the bob and the height of the bob
above the tabletop, the potential energy of the bob can be determined.
Since the gravitational potential energy of an object is directly proportional to its height above
the zero position, a
doubling
of the height will result in a
doubling
of the gravitational
potential energy. A
tripling
of the height will result in a
tripling
of the gravitational potential
energy.
