Defining Energy
The historical development of the concept of energy was long and laborious. It took more than 150 years
m the first attempts at quantitative formulation, by the Dutch contemporary of Galileo, Descartes, and ewton, Christian Huygens (1629-1695), to the point, which the appropriate terminology was established. In general, energy does not have properties like those atter, such as size, shape, and color. Also unlike matter, it cannot in general be said to occupy space. Nevertheless, matter is but one more manifestation of energy so that we must qualify our statements by saying "in general.
How then do we define this somewhat abstract concept? We can say that energy is a measure of the of a physical system to perform work when the system undergoes a change. ("Change" implies that we should be able to describe the system accurately before and after in order to be able to say that it has changed.) Yet change alone is not a sufficient means of defining energy: As human beings, we are physical systems, but changing our feelings for other human beings does not perform work. A genuine example of energy is what happens when a stream turns a waterwheel as it flows over a dam; the stream performs work on the waterwheel, which rotates a grindstone, which grinds grain. The energy of the stream is a measure of the ability of the stream to perform useful work
Although energy does not in general have the properties of matter, it can be measured and quantified. One unit used by astronomers is the erg, the amount of energy needed to accelerate a mass of 1 gram at a rate of 1 centimeter per second squared as it moves a distance of 1 centimeter.
