The Question "What is Physics?" has a long history and even a longer answer when you consider that to ask the question demands that you look at the history of humanity's knowledge of the physical world.

IIT (the Illinois Institute of Technology) has a small poster that they sent out to science teachers that suggested that to be educated in science and to teach science in a logical order you had to build a strong intellectual structure.  They selected the pyramid to best represent this structure.  The base of the pyramid is mathematics, the foundation of all science.  It is the language used to describe patterns and relationships among variables and is used to create models of things often too abstract to contemplate in any other medium of thought.  It is suggested by some very intelligent persons that mathematics is the language of the architecture of the universe.

Upon that base is the study of physics so that one may learn the laws and principles that govern all matter.

Third from the bottom is the study of chemistry where one may learn about the characteristics and behavior of all matter.

Finally at the pinnacle of the study of science is biology where the pursuit of the characteristics and behavior of living matter is studied.  The significance of this model is that some very perceptive people came to realize that to create an environment where students came to understand science they had to study it and investigate in the order which would permit logical progression of the mind.  This means that after mathematics, physics is the next science to be pursued, if real understanding is to come of studying the sciences.  Unfortunately, in many schools mathematics and the sciences are pursued out of order creating confusion and difficulty for both the teacher and the student.

Isaac Asimov, a hero of mine and many others, wrote a tremendously interesting work which has appeared in print as a large book called "Understanding Physics"  Three Volumes in One, by Barnes and Noble (copyright 1966 by Isaac Asimov) last edition published in 1993, ISBN 0-88029-251-2.

In the beginning of the first volume he writes:

"The scholars of ancient Greece were the first we know of to attempt a thoroughgoing investigation of the universe - a systematic gathering of knowledge through the activity of human reason alone.  Those who attempted this rationalistic search for understanding, without calling in the aid of intuition, inspiration, revelation, or other non rational sources of information, were the philosophers (from Greek meaning "lovers of wisdom").

Philosophy could turn within, seeking an understanding of human behavior, of ethics and morality, of motivation, and responses.  Or it might turn outside to an investigation of the universe beyond the intangible wall of the mind - an investigation, in short of nature.

Those philosophers who turned toward the second alternative were the natural philosophers, and for many centuries after the palmy days of the Greeks the study of the phenomena of nature continued to be called natural philosophy.  The modern word that is used in its place - science, from a Latin word meaning "to know" did not come into use until well into the 19th century.  Even today the highest university degree given for achievement in the sciences is generally that of "Doctor of Philosophy.

The word "natural" is of latin derivation, so the term "natural philosophy" stems half from the Latin and half from the Greek a combination usually frowned on by purists.   The Greek word for natural is physikos, so one might more precisely speak of "physical philosophy" to describe what we now call science.

The term physics, therefore, is a brief form of physical philosophy or natural philosophy and, in its original meaning, included all of science.

However, as the field of science broadened and deepened and as the information gathered grew more voluminous, natural philosophers had to specialize, taking one segment or another of the scientific endeavor as their chosen field of work.  The specialties received names of their own and were often subtracted from the once universal domain of physics.

Thus, the study of the abstract relationships of form and number became mathematics; the study of position and movements of the heavenly bodies became astronomy; the study of the physical nature of the earth we live on became geology; the study of the composition and interaction of substances became chemistry; the study of the structure, function, and interrelationships of living organisms became biology, and so on.

The term physics then came to be used to describe the study of those portions of nature that remained after the above - mentioned specialties were subtracted.  For that reason the word has come to cover a rather heterogeneous field and is not as easy to define as it might be.

What has been left over includes such phenomena as motion, heat, light, sound, electricity, and magnetism.  All these are forms of energy ... , so that a study of physics may be said to include, primarily, a consideration of the interrelationships of energy and matter.

This definition can be interpreted either narrowly or broadly.  If it is interpreted broadly enough, physics can be expanded to include a great deal of each of its companion sections of science.  Indeed the twentieth century has seen such a situation come about."

Many textbooks have made an attempt to squeeze a definition into a single statement.  If you were to do that, Isaac Asimov's sentence describing physics as being "a consideration of the interrelationships of energy and matter" would be a good statement to begin with.