Harvey Avery, age 14, of Gadsden, Ala., for his question:

WHAT EXACTLY IS PHYSICS?

Physics is the science that deals with matter, energy, motion and force. It tells us the "how" and "why" of the nonliving world. The term physics comes from a Greek word meaning nature.

Physics tells us why water freezes and evaporates. It also tells us how radio and television work, how eyeglasses correct faulty vision and why a ball bounces.

Seven main groups of subjects are studied by physicists: mechanics, heat, light, electricity and magnetism, sound, atomic and nuclear physics and solid state physics.

Mechanics is the study of objects, forces and motion and such properties of matter as elasticity and weight. It deals with the forces that make an airplane fly and keep an automobile on a highway when speeding around a curve.

The study of heat is called thermodynamics. Physicists study how heat can be changed into work, how it is produced, how it is transferred from one place to another and how it changes matter. Heat ranks as one of the most important kinds of energy that man uses.

The study of light is called optics. It involves learning what light is, how it behaves and how it can be used.

Electricity and magnetism are closely related. One of the most important advances in science has been the development of electronics, the branch of physics that studies the behavior of electrons.

The study of sound is called acoustics. It explains how sound is produced, transmitted and reflected, and why it changes.

Atomic and nuclear physics have revealed many of the secrets of matter, while solid state physics has supplied information that has led to such developments as the transistor and the solar battery.

Using deduction and logic rather than experiments and observations, Aristotle in the 300s B.C. formed theories in many areas of physics.

Then in the 200s B.C., Archimedes discovered laws for the behavior of levers and of liquids.

Galileo in the 1600s discovered important laws in many fields of physics, especially mechanics. He was the first great experimenter and also called the father of modern astronomy.

Many important steps in physics weren't taken until the 20th century came along. In 1905, Albert Einstein published his Special Theory of Relativity and 10 years later announced his General Theory of Relativity.

In 1938, Otto Hahn and Fritz Strassmann achieved fission of uranium and in 1942, Enrico Fermi and his associates achieved the first controlled nuclear chain reaction.