Work: The Transfer of Mechanical Energy

There are many situations in which we would like to know how much energy comes in or out of a system that is not closed. Energy, however, is not a physical substance like water, so energy transfer cannot be measured with the same kind of meter.

The law of conservation of energy guarantees that all the chemical energy in the gasoline will reappear in some form, but not necessarily in a form that is useful for doing some work. Tractors or cars, are extremely inefficient, and typically about 70 - 90% of the energy they consume is converted directly into heat, which is carried away by the exhaust and the air flowing over the radiator. We wish to distinguish the energy that comes out directly as heat from the energy that served to accelerate a car, and we define a technical meaning of the ordinary word “work” to express the distinction: