Energy

The sum total of an object's potential and kinetic energy is known as its mechanical energy. The total amount of mechanical energy possessed by a body is a constant. The baseball described above has a maximum potential energy and minimum kinetic energy (actually a zero kinetic energy) while at rest. In the fraction of a second before the ball has struck the ground, its kinetic energy has become a maximum and its potential energy has reached almost zero.

The case of the falling baseball described above is a special interest of a more general rule known as the law of conservation of energy. According to this law, energy can never be created or destroyed. In other words, the total amount of energy available in the universe remains constant and can never increase or decrease.

Although energy can never be created or destroyed, it can be transformed into new forms. In an electric iron, for example, an electrical current flows through metallic coils within the iron. As it does so, the current experiences resistance from the metallic coils and is converted into a different form, heat. A television set is another device that operates by the transformation of energy. An electrical beam from the back of the television tube strikes a thin layer of chemicals on the television screen, causing them to glow. In this case, electrical energy is converted into light. Many of the modern appliances that we use in our homes, such as the electric iron and the television set, make use of the transformation of energy from one form to another.

In the early 1900s, Albert Einstein announced perhaps the most surprising energy transformation of all. Einstein showed by mathematical reasoning that energy can be converted into matter and, vice versa, matter can be transformed into energy. He expressed the equivalence of matter and energy in a now famous equation, E = m × c², where c is a constant, the speed of light.