Dictionary Definition of Energy
The capacity for vigorous activity. The measure of energy indicates the amount of work an object is capable of taking. A unit of energy is [J]: joule, the same as a unit of work.
Physical energy is a capacity for work. What it means by work here is physical work defined by multiplication of migration lengths of two forces operating toward certain directions.
If an object in a high place falls, it works by applying force to another object, and an object moving in a certain velocity can work by applying force to other objects that obstruct its movement. For example, we can move a mill wheel by dropping water from a higher place and a moving vehicle can push a stationary vehicle by a collision. Like this, an object in a high place or an object moving in a certain velocity has energy that can work.
To quantify such energy, an object in a high place is defined as potential energy (gravitational) that is relative to the height from a reference point and its mass and a moving object as kinetic energy that is relative to the square of its velocity and mass.
Besides gravitational potential energy, the definition of potential energy can be applied to elastic potential energy that is stored in an elastic body by its distortion, electric potential energy (usually called voltage or electric potential). The sum of potential energy and kinetic energy is called mechanical energy.
In addition, there are many types of energy in which the measure is determined by diverse kinds of physical conditions in the natural world, such as thermal energy, chemical energy, sound energy, and light energy.
According to Einstein’s theory of relativity, mass and energy are equivalent. Therefore, when the mass of an object decreases, corresponding energy is yielded. It generates nuclear power using the decrease of mass that is yielded from its nuclear fusion process.