In physics, energy is an indirectly observed quantity which comes in many forms, such as kinetic energy, potential energy, radiant energy, and many others; which are listed in this summary article. This is a major topic in science and technology and this article gives an overview of its major aspects, and provides links to the many specific articles about energy in its different forms and contexts.
The question "what is energy?" is difficult to answer in a simple, intuitive way, although energy can be rigorously defined in theoretical physics. In the words of Richard Feynman, "It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount."
However, it is clear that energy is always an indispensable prerequisite for performing mechanical work, and the concept has great importance in natural science. The natural basic units in which energy is measured are those used for mechanical work; they always are equivalent to a unit of force multiplied by a unit of length. Other equivalent units for energy are mass units multiplied by velocity units squared.
Energy exists in many forms:
In the context of physical sciences, several forms of energy have been defined. These include:
These forms of energy may be divided into two main groups; kinetic energy and potential energy. Other familiar types of energy are a varying mix of both potential and kinetic energy.