Mechanical Energy, The Energy Of Work
A simple definition of energy is that energy is the ability to cause change. If there's a thing that can change his chemical of physical properties, then that object has energy. Even motion of things includes a certain kind of energy. It is called mechanical energy.
When a force is applied on an object, work is done. Every change of motion implies work. Even when we put a book on the shelf, when we throw a ball or when we wash the dishes, we apply force to do those things. We are then in the presence of mechanical energy. A few centuries ago people used water to turn the wheels in a river, or steam to power turbines. Even the gears of a wind up clock need mechanical energy to work.
There are two types of mechanical energy. The kinetic energy which is the energy of motion, and the potential energy, also called stored energy. The sum of these types of energy represents the total mechanical energy of an object. For a better understanding of these terms, I'll give you some examples. Let's consider a rock on the edge of a cliff and water at the top of a dam. Both the rock and the water have mechanical potential energy, because of their very position that allows them to fall freely. But, if you push the rock and open the gate of the dam, the potential energy will become mechanical kinetic energy. The fall of the rock and the flow of the water allow the change of energy. So, we can say that the energy of position changes into the energy of motion. And, these are just two of the many examples that can be given of objects with mechanical energy.
We use mechanical energy every day. Let's take for example the hammer. Mechanical energy gives the hammer the power to apply force on a nail. Even the motion of wind or water represents forms of mechanical energy. People used it for ages, as windmills, to raise water from wells, and for many other purposes. Even now, in some areas of U.S. there are wind farms that use the mechanical energy of wind to obtain electricity.
So, whenever force acts upon an object, whenever we act upon a thing to cause its displacement, we have mechanical energy. This is the reason for which mechanical energy is called the ability to do work.