Now that we understand the concept of work, we can use it to define energy. Anything which is capable of doing work is said to possess energy. A falling water has energy because it can do work by turning the turbine of a generator. A battery has energy because it can rotate an electric motor that does work. Wind has energy because it can turn the blades of wind turbines. From these examples, we can define energy as follows:

• Energy is the ability to do work.

Energy comes in differnt forms. Mechanical energy, electrical energy, chemical energy, nuclear energy, and light energy are some of the forms of energy. In this lesson, however, we shall study only mechanical energy, which consists of two distinct types: (i) kinetic energy (KE), associated with the motion of an object; and (ii) potential energy (PE), associated with the position of an object relative to the ground or some other bodies taken as a reference.

Kinetic Energy (KE) is the ability of an object to do work because of its motion. As shown in Figure 5, the moving hammer can do work on the nail driving it into the wall because it has kinetic energy. If an object of mass m moves with a velocity v, its kinetic energy is given by: