Energy is known as the ability to do work , But where did this definition come from ? In this Article some of the basics physics of energy will be discussed and explained briefly.

**What Is Energy !?**

Energy. *E* is defined as the ability to do work, *W *.work is the consequence of the expenditure of energy and is defined as the product of a force, *F*.acting on an object times the distance, *d*,That the object moves This relationship could be written as

** W=Fd **(1-1)

This expression assumes That the force acting on the object is a constant over the time during which the object moves a distance, *d*,and that the force is acting in the same direction as the displacement.

The unites of the work are the same as the unites of energy. In the metric system, the standard unite of energy is joule *(J)* When the force is expressed in newtons and the displacement in meters *(m)*.

The British unit of energy , which is often used in engineering in the united states, is the British thermal unite *(Btu , 1 Btu =1055 J)*, The Btu is commonly used to designate thermal energy.

It is perhaps convenient to think of the concept expressed in equation (1-1) in terms of a mechanical System. An object with a mass,* m*,lying at rest on the floor exerts a force (The gravitational force), *mg, g* is the gravitational acceleration, downward on the floor.

The floor exerts a force, *mg*, upward on the mass (the normal force that cancels out the gravitational force, the net vertical force on the object is zero and from Newton’s law

** F=ma **(1-2)

The acceleration is zero, and the object does not move. The work done is zero because the distance that the object travels is zero. if an external vertical force that is equal to or greater than *mg* is exerted on the object. Then the object can be lifted from the floor. if the object is lifted to a height, *h*, then the work done from equation (1-1), is the force times the distance ,or

** W=mgh **(1-3)

According to the law of conservation of energy, this work is converted into gravitational potential energy. also equal to *mgh*. the work is done is independent of how long the process takes or of the path taken to reach height *h*. because of his later property , the gravitational force is said to be conservative.

It is sometimes convenient to deal with power rather than with energy or work. Power ,*P*, is the rate at which work is done (or the rate a which energy is expended). Power is measured in watts* (w)* , and that watt is defined as 1 joule per second , The British unite of power is horsepower *(hp)* where *1 hp = 746 W* . Assuming that power is a constant in time ,*t*, Then the total energy utilized is

** E=Pt ** (1-4)

This definition shows that 1 W.s = 1 J. Total energy is the power integrated over time So the producing or using 1000 w of power for 1 second represents the same amount of energy as producing or using *1 w* for 1000 second.

Equation (1-4) provides the basics for an alternative unite for the measurement of energy, the kilowatt-hour *(KWh). *The KWh is defined as the energy corresponding to a power of 1 kilowatt (1000 W) over a period of 1 hour (3600 s) so that 1KWh = 1000 W * 3600 s = 3.6 * 10 * 10* 10* 10* 10* 10 J.

**Classifications Of Energy Forms **

Energy May be categorized in different ways . One approach is to classify energy as either Kinetic or potential. Form a classical point of view , Kinetic energy is merely the energy associated with the motion of a body.

Potential energy may be described in terms of the nature of the interactions in a System, For example, gravitational potential energy of water in an elevated reservoir maybe converted into kinetic energy, which can be utilized to turn a turbine.

**Source
**Sustainable Energy (Paperback) By (author): Richard Dunlap