Potential Energy
Directions: read and highlight and then answer the questions at the end.
Different types of Energy
Recall, energy can be broken down into two basic forms. Since energy is the ability to do work, we
defined it using the term work. Energy can be thought of as both “work waiting to happen” and “work
happening.” Work “waiting to happen” means that something might do work. Work that is already
happening is in motion. So, we have two kinds of work. Work waiting to happen and work in motion.
Kinetic Energy
Work in motion uses something we have already been learning, speed. Recall, we said that speed deals
with an object being in motion. So if we combine these two terms, we have an object in motion doing
work. We refer to this as “energy in motion” or the science term, kinetic energy.
Work Waiting to Happen
In energy terms, the “work waiting to happen” is called potential energy. We define potential energy
(PE) as the energy of position. In most of our examples the position refers to how high in the air
something is. Although we don’t talk about these in 6th grade, there are many types of potential energy
that explain, for example, how springs work or even how chemicals work.
Gravitational Potential Energy
Energy of position has a special name, gravitational potential energy. This type of potential energy
depends on an object’s mass, height, and gravity. For example, when you lift a book into the air, this
book will have potential energy because it has height from being lifted off the ground.
KE vs. PE
Potential energy is different from kinetic energy because it can be stationary or moving but kinetic
energy has to be moving. For example, a book sitting on a shelf has lots of potential energy but has no
kinetic energy.
Calculating Gravitational Potential Energy
The formula for calculating gravitational potential energy is as follows:
Potential Energy, PE = gravity x mass x height
Gravity refers to the force of gravity pushing down on all of the
objects on earth. Depending where you are in the solar system,
gravity can change, but here on good old earth, gravity is
approximately the same everywhere, 9.8.
What about the height? Height above what? For example, if I am
standing on a desk at school (breaking science classroom rules ), is
my height in relation to the floor of the classroom or outside on the
ground? Basically, it’s the closest point where you will stop falling.
If I am standing on a desk, this would be the classroom floor. If I was
jumping off the desk and out the window, my height would be
measured from the top of the desk down to the ground. So
understanding my “ground” location is extremely important for
potential energy.
An Example
Let’s do an example. If I am standing on a desk 1 m off the floor and weigh 100 kg, what is my PE? Just
as we did with kinetic energy, let’s set up the problem.
PE
= gravity x mass x height
= 9.8 x 100 kg x 1 m
= 980 J
Just as kinetic energy is measured in joules (J) so is potential energy.
What if I was standing on the desk and jumping out the window (my height is now 21 m)?
PE = gravity x mass x height = 9.8 x 100 kg x 21m = 20,580 J
So just by increasing my height above ground, the PE increases up to 20,580 J.
Questions
1. Write the formulas for each type of energy:
a. KE =
b. PE =
2. Name one similarity between the formula for potential energy and the formula for kinetic energy.
3. Name a difference (other than KE has ½ in its formula and PE doesn’t) between the formula for
potential energy and the formula for kinetic energy
4. Name two ways that you can increase your potential energy while still on earth.
a.
b.
5. Calculate the potential energy if your weight is 50 kg and you
jump from a tree swing reaching a height of 5 meters off the
ground.
6. Once you land on the ground, what is your new potential energy?