Structure of Matter

CONCEPT The physical realm is made up of matter. On Earth, matter appears in three clearly defined forms—solid, liquid, and gas—whose visible and perceptible structure is a function of behavior that takes place at the molecular level. Though these are often referred to as “states” of matter, it is also useful to think of them […]

Molecular Dynamics

Concept Physicists study matter and motion, or matter in motion. These forms of matter may be large, or they may be far too small to be seen by the most high-powered microscopes available. Such is the realm of molecular dynamics, the study and simulation of molecular motion. As its name suggests, molecular dynamics brings in […]

Gas Laws

Concept Gases respond more dramatically to temperature and pressure than do the other three basic types of matter (liquids, solids and plasma). For gases, temperature and pressure are closely related to volume, and this allows us to predict their behavior under certain conditions. These predictions can explain mundane occurrences, such as the fact that an […]



Concept As with many concepts in physics, energy—along with the related ideas of work and power—has a meaning much more specific, and in some ways quite different, from its everyday connotation. According to the language of physics, a person who strains without success to pull a rock out of the ground has done no work, […]

Mechanical Advantage and Simple Machines

Concept When the term machine is mentioned, most people think of complex items such as an automobile, but, in fact, a machine is any device that transmits or modifies force or torque for a specific purpose. Typically, a machine increases either the force of the person operating it—an aspect quantified in terms of mechanical advan-tage—or […]

Work and Energy


CONCEPT Unlike fluids, solids do not respond to outside force by flowing or easily compressing. The term elasticity refers to the manner in which solids respond to stress, or the application of force over a given unit area. An understanding of elasticity—a concept that carries with it a rather extensive vocabulary of key terms—helps to […]


CONCEPT Pressure is the ratio of force to the surface area over which it is exerted. Though solids exert pressure, the most interesting examples of pressure involve fluids—that is, gases and liquids—and in particular water and air. Pressure plays a number of important roles in daily life, among them its function in the operation of […]


CONCEPT Statics, as its name suggests, is the study of bodies at rest. Those bodies may be acted upon by a variety of forces, but as long as the lines of force meet at a common point and their vector sum is equal to zero, the body itself is said to be in a state […]