Name: ___________________________________
Potential Energy
Hopefully you can remember from 6th grade that an object can store energy
because of its position. For example, the wrecking ball of a crane is storing energy
when it is held above the ground 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
it’s at the usual position (i.e., when not drawn), there is no energy stored in the
bow. But when it’s pulled back, the bow is able to store energy because of its
position. This stored energy of position is also referred to as potential energy. In this case, the bow
represents a special form of potential energy called elastic potential energy.
Elastic Potential Energy
The first type of potential energy that we will explore is elastic
potential energy. Elastic potential energy is the energy stored in
elastic materials as the result of their stretching or compressing. An
elastic material is a material that can resume its normal shape after
being stretched.
Elastic potential energy can be stored in a variety of materials such as
rubber bands, bungee cords, trampolines, springs, an arrow drawn
into a bow, etc. The amount of elastic potential energy stored in such a device is related to the amount of
stretch - the more stretch, the more stored energy.
If a rubber band (or any elastic material) is not stretched, then there is no elastic potential energy stored
in it. The rubber band is said to be at its equilibrium position. The equilibrium position is the position that
the rubber band naturally assumes when there is no force applied to it. In terms of elastic potential
energy, the equilibrium position could be called the zero-potential energy position.
Like most things in physics, there is an equation for elastic materials that relates the amount of elastic
potential energy to the amount of stretch (or compression) and the constant. The equation is:
PEelastic = 0.5 • k • x2
where k = spring constant (you will learn how to do calculate this)
x = amount of stretch beyond the equilibrium position
To use this equation, I will initially give you the spring constant and the amount of stretch (x). Let’s do an
example:
If a rubber band has a stretch of 0.1 meters past equilibrium and a spring constant of 40, determine the
elastic PE:
PE = 0.5 * 40 * (0.12) = 20 * .01 = 0.2 joules (J)
If you recall from 6th grade, PE is measured in joules which is the symbol J.
To summarize, potential energy is the energy that is stored in an object due to its position relative to
some zero position. An object possesses elastic potential energy if it is stretched (beyond the equilibrium
position).
Questions:
1. Go to schoology.com and answer the questions on elastic potential energy (located in the PE folder).