We are all familiar with the idea of
energy, but it was highly debated until
the 1850’s. Energy is what sustains life,
but yet it is still difficult to define.
Perhaps because energy is only
observable when it is being transferred
from one place to another, or
transformed from one type to another.
We will begin energy by looking at a
related concept, work.
The change in an objects motion is
related to both the force that acts upon
it, and the amount of time that the force
acts on the object. In this instance, time
is a factor.
When we study work, time is not a
variable. Work uses the quantities of
force and distance.
When we do work, we lift a load against
earth’s gravity. The heavier the load, or
higher we lift it, the more work that is
Two variables are involved in every case
of work being done:
 the application of a force
 the movement of something
Work = force x distance
If we lift two loads up 1 story of a
building, we do 2x as much work as
lifting only one load up the same
distance, because the force required
was 2x as great as only one trip.
Similarly, if we lift one-load two stories
as opposed to one, we do 2x the work,
because the distance is 2x as great.
Work involves both a force and a
A weight lifter holding a barbell above
his head does no work on the barbell,
because it is not moving. He is doing
work on his muscles, which are
stretching and squeezing. The initial lift
of the barbell, however, was/is work.
There are generally two categories of
work, one is the work done against
another force, for example pulling back
a bowstring. The other is work done to
change the speed of an object, for
example speeding it up, or slowing it
Work combines Newtons (force) with
meters (distance) to give us the joule.
One joule of work is done when 1N is
exerted over 1m.
Work does not deal with time. To
discuss how fast work is done, we use
power. Power is the rate at which work
is done.
Power= work/time
High-powered engines do work rapidly,
and therefore have a lot of power.
The unit of power is the watt.
One watt of power is used when one
joule of work is done is one second.
A kilowatt is 1000 watts.
In the U.S. we customarily rate engines
using horsepower, but the metric system
of rating engines uses kilowatts. One
horsepower is the same as 0.75
Mechanical Energy
Energy is also measured in joules. It is
that certain something that allows an
object to do work. It has many forms.
One of the most common is mechanical
This is the energy due to the position of
an object, or the movement of an object.
There are two types of mechanical
energy: kinetic and potential.
Potential Energy
Energy that is stored and held is
potential energy. Examples are a
stretched rubber band, a drawn back
bow, a boulder at the top of a hill, or a
paused roller coaster at the top of a hill.
All types of fuel contain potential energy,
such as batteries, fossil fuels, and food.
Elevated objects that would fall because
of gravity have gravitational potential
energy. Example: water tower, pile
drivers ram.
The gravitational energy possessed by
an object is equal to the amount of work
it took to lift it against gravity.
We can figure potential energy by
multiplying the weight by the height.
 PE=weight x height
Kinetic Energy
If an object is moving, it can do work.
This energy of motion is called kinetic
energy. Kinetic energy is dependent on
the mass of an object as well as its
 KE= ½ mass x speed2 or, KE = ½ mv2
When you throw a ball, it has the ability
to hit something and push it, therefore
doing work on that object.
If there is no change in energy, no work
occurs. This is the work-energy
Conservation of Energy
It is important to understand how energy
transforms from one type to another more than
what energy really is.
Imagine this scenario:
As you draw back the stone in a slingshot,
you do work stretching the rubber band. The
rubber band then has potential energy. When
released, the stone has kinetic energy equal to
this potential energy. It delivers this energy to
a target. Energy changes from one form to
another without loss or gain.
The law of conservation of energy says:
Energy cannot be created nor
destroyed. It can be transformed
from one form to another, but the
total amount of energy never