Energy is the ability to move or change
Often, energy is defined as the “ability to do work”, but this description of energy is fairly
simplistic and not very precise. However, many use this description because it is almost impossible to define
energy in just a few words.
A better description is that energy is the ability to move or change something. Although this still has its
problems it gives us a better picture of energy.
The SI unit for energy is the joule (J) named after the English physicist, James Prescott
Joule. A joule is equal to a newton moving through a distance of one meter.
1 joule = 1 N · m
Energy can be neither created nor destroyed, but it can be converted from one form to another
and it can be transferred from one object to another.
Energy can be stored in an object because of it's position or it's motion.
Potential energy is "stored energy" due to either an object's position, chemical composition or
Gravitational potential energy (PE) is the energy an object has with respect to it's mass
and height. The potential energy is directly proportional to both the mass (m) and height (h). (g is gravity)
PE = mgh
The potential energy of a spring is refered to as elastic potential energy (U) and can be
determine using Hooke's law (F=kx).
(k is the
The elastic potential energy of a spring is one half the product of the spring constant and the square of the
Kinetic energy is "stored energy" due to an object's motion. Kinetic energy (KE) is the energy of
motion and is directly proportional to the mass and the square of velocity of the object.
KE = ½mv2
Conservation of Energy
If a system is frictionless and makes no sound potential energy will be completely converted into kinetic energy
and kinetic energy is completely converted into potential energy. So, the energy of the system is