http://www.ck12.org Chapter 2. Energy ConservationVersion 2
2.2 Types of Energy
Understanding how various processes change energy from one form to another is equivalent to understanding
physics. In this class, we will present an overview of various forms of energy but will mainly focus on three:
kinetic, gravitational potential, and electrical potential. We will focus on the first two in this chapter; electrical
potential energy is covered in later chapters.
Kinetic Energy
The first is Kinetic Energy, or the energy of motion. Any moving object — from the earth to an individual gas
molecule — has some kinetic energy, which can be calculated by using the following formula:
K=^12 mv^2 [1]Themrefers to the object’s mass, while thevis its speed.
Gravitational Potential Energy
The second type of energy is due to gravity and is therefore called gravitational potential energy. Things with mass
have noticeable gravitational potential energy when they are near another object of significant mass, such as the earth,
the sun, a black hole, etc. This energy is different from kinetic energy in that it represents potential for motion, rather
than motion itself. If I lift a rock away from the surface of the earth to some height and then let it drop, it will gain
velocity as it travels downwards. According to the last paragraph, this means it also gains kinetic energy. Assuming
no energy is lost to air resistance, there will be a one to one correspondence between gravitational potential energy
lost and kinetic energy gained. Near the surface of a planet, the gravitational potential energy gained by an object of
massmwhen raised a height∆hfrom its original positionperpendicular to the surface of the planetis just
Eg=mg∆h[2]The constantgwill vary from planet to planet, star to star. On earth, the acceleration due to gravity is 9.8m/s^2 , often
rounded to 10m/s^2. This is the formula you will likely use the majority of the time. However, there is a way to
express the gravitational potential energy ofany two objects in the universe— any number of objects, in fact. For
the two object case, if we call their massesm 1 andm 2 and the distance between theircenters of mass r, the formula
is:
EG=
Gm 1 m 2
r[3]
In fact, equation [2] is aspecial caseof equation [3]. That is, it is a version of equation [3] that holds under specific
circumstances. See the appendix to this chapter for a derivation.