what actually energy is?

Answer 1

Energy is defined as the ability to do work.

E=mc^2
Potential energy= mgh (g=gravity, h=height)
Kinetic energy=1/2 mv^2 (v=velocity, m=mass in both )

Answer 2

energy, in physics, the ability or capacity to do work or to produce change. Forms of energy include heat, light, sound, electricity, and chemical energy. Energy and work are measured in the same units—foot-pounds, joules, ergs, or some other, depending on the system of measurement being used. When a force acts on a body, the work performed (and the energy expended) is the product of the force and the distance over which it is exerted.

Potential and Kinetic Energy

Potential energy is the capacity for doing work that a body possesses because of its position or condition. For example, a stone resting on the edge of a cliff has potential energy due to its position in the earth's gravitational field. If it falls, the force of gravity (which is equal to the stone's weight; see gravitation) will act on it until it strikes the ground; the stone's potential energy is equal to its weight times the distance it can fall. A charge in an electric field also has potential energy because of its position; a stretched spring has potential energy because of its condition. Chemical energy is a special kind of potential energy; it is the form of energy involved in chemical reactions. The chemical energy of a substance is due to the condition of the atoms of which it is made; it resides in the chemical bonds that join the atoms in compound substances (see chemical bond).

Kinetic energy is energy a body possesses because it is in motion. The kinetic energy of a body with mass m moving at a velocity v is one half the product of the mass of the body and the square of its velocity, i.e., KE = 1/2mv2. Even when a body appears to be at rest, its atoms and molecules are in constant motion and thus have kinetic energy. The average kinetic energy of the atoms or molecules is measured by the temperature of the body.

Answer 3

Energy is everything in this universe, at least according to Einstein. He (or his wife, as some have proposed) came up with the famous equation, E=mC^2, relating energy to mass. In this view, mass is nothing more than "frozen energy". It is that principle that led to nuclear weapons such as the atomic bomb and the hydrogen bomb, along with nuclear power plants and the source of heat that drives mantle circulation.

Answer 4

Energy is mass times the speed of light squared.

E=MC^2

Mass is anything that has weight and takes up space.

Speed of light is 186,000 miles per second.

Answer 5

Energy (from the Greek ἐνέργεια, energeia, "action, act, work") is an important concept in science, with its origins in physics via work, and it is very convenient quantity which thus finds applications throughout the natural sciences.

Energy is subject to a strict global conservation law; that is, it can neither be created nor destroyed. This law mathematically follows from shift symmetry of time (no moment of time is different than any other) and mathematical definition of energy.

The total energy of a system can be subdivided and classified in various ways. For example, it is sometimes convenient to distinguish potential energy (which is a function of coordinates only) from kinetic energy (which is a function of coordinate time derivatives only). It may also be convenient to distinguish gravitational energy, electrical energy, thermal energy, and other forms. These classifications overlap; for instance thermal energy usually consists partly of kinetic and partly of potential energy.
The transfer of energy can take various forms; familiar examples include work, heat flow, and advection, as discussed below.
The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency. The vernacular terminology is not consistent with technical terminology. For example, the important public-service announcement, "Please conserve energy" uses vernacular notions of "conservation" and "energy" which make sense in their own context but are utterly incompatible with the technical notions of "conservation" and "energy" (such as are used in the law of conservation of energy).
In classical physics energy is considered a scalar quantity, having no direction in space. In special relativity energy is also a scalar (although not a Lorentz scalar but a time component of the energy-momentum 4-vector). In other words, energy is invariant with respect to rotations of space, but not invariant with respect to rotations of space-time (= boosts).

Definitions
Energy, in physics, is defined as the amount of work a physical system can do on another.

For a general audience, rather than worrying about the details of a formal definition, it is far easier and far more useful to understand what energy does in various situations. This is called the "energy is as energy does" school of thought. This approach emphasizes examples of energy, the strict local conservation of energy, and the connection between energy and other quantities of interest. Some textbooks take the view that energy is primary and fundamental, and introduce it as a thing in itself, without relying on prior definitions of force or work or momentum. Other textbooks first mathematically define work (or other equivalent path integral like action), and then mathematically define energy via work (or action)).

Conventional definition of "energy" is the capacity to do work. This definition is prevalent in textbooks, dictionaries, and other technical references.This notion of "energy" (or, rather, "available energy") is extremely useful and is used to calculate practically any kind of energy (usually calculated energy is named after work of certain force - gravitational energy, electric energy, elastic energy, etc.)

The above definition can be extended to the amount of work one system can do on another that is, the capability of a system to change another That is the usual definition of energy in physics, in particular.


Historical perspective
Main articles: Timeline of thermodynamics, statistical mechanics, and random processes and History of Physics
The concept of energy, a long time ago, was used to explain easily observable phenomena, such as the effects observed on the properties of objects. It was generally construed that all changes could in fact be explained through some sort of energy. Soon the idea that energy could be stored in objects took its roots in scientific thought and the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms. While in spiritualism they were reflected in changes in a person, in physical sciences it is reflected in different forms of energy itself, for example, electrical energy stored in a battery, the chemical energy stored in a piece of food (along with the oxygen needed to burn it), the thermal energy of a water heater, or the kinetic energy of a moving train. In 1807, Thomas Young was the first to use the term "energy" instead of vis viva to refer to the product of the mass of an object and its velocity squared. Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined the term "potential energy."

The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Clapeyron and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contributed to the notion that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum.

William Thomson (Lord Kelvin) amalgamated all of these laws into the laws of thermodynamics, which aided in the rapid development of explanations of chemical processes using the concept of energy by Rudolf Clausius, Josiah Willard Gibbs and Walther Nernst. It also led to a mathematical formulation of the concept of entropy by Ludwig Boltzmann, and to the introduction of laws of radiant energy by Jožef Stefan,

During a 1961 lecture[4] for undergraduate students at the California Institute of Technology, Richard Feynman, a celebrated physics teacher and Nobel Laureate, said this about the concept of energy:

There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law—it is exact so far we know. The law is called conservation of energy [it states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes]. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same.

– The Feynman Lectures on Physics, Vol. 1.

Since 1918 it is known that energy conservation law is the direct mathematical consequence of translational symmetry of conjugate to energy quantity - time (no moment of time is any different than any other) - see conserved currents in Emmy Noether theorem