Dynamics is the study of the behavior of an object when there is a force (a push or a pull) acting on it. Every object on earth or in space can be classified as a solid, a liquid, or a gas.

Solid objects have well-behaved molecules and atoms. These molecules line up in an even pattern that gives the object a specific shape. A block of wood is a solid; so is a crystal of salt. The primary characteristic of a solid is that the shape stays fixed. If a cylinder of wood is placed in a square container, its shape doesn't change to match the container. In dynamics, this is called a non-deformable body. A secondary characteristic of a solid is that no matter how hard it is squeezed or pulled, the molecules do not move closer together or further apart. The object may break, but the molecules don't move. This is called an incompressible object.

The molecules in a liquid, however, are not so well-organized. An amount of fluid, when poured from a cylindrical container into a round one, will not retain its cylindrical shape. It will take the shape of the round container. A primary characterization of a liquid is that it will deform, or take the shape of its container, but it will not change the amount. A large cylinder of a liquid, for example water, poured into a small round bowl will likely overflow the bowl. Likewise, a small cylinder of liquid, like a glass of water, poured into a large round bowl will only fill a part of the bowl. Like the solid, however, if a liquid is squeezed or pulled, the molecules do not move closer or further apart. It, too, is an incompressible object.

Gases like air have even less-organized molecules. Gases not only will take the shape of their containers, but also will expand or contract to fill the container. When a person takes a breath of air, for example, the air rushes down the bronchial tubes and tries to fill all the spaces in the lungs. A big breath makes it easier to feel the lungs expand, but a small breath fills ALL of the lungs, too. An object made of a gas is a deformable, expandable body. This expandable characteristic is also called compressible.

A fluid object can be either a liquid or a gas. The primary characterization, or property, of a fluid is that it is a deformable body. Sometimes a large quantity of a solid can show fluid properties; it can act like a liquid. A handful of sand, for example, is made up of solid grains of sand. But if it is poured into a cylinder, the handful will take the shape of the cylinder! The behavior of a large amount of sand, then, would follow both the laws of solid dynamics and fluid dynamics!

The study of dynamics, then, can be categorized into four specialties: dynamics of solids, which will be discussed more in the structures chapter, how liquids behave (hydrodynamics), how air and other gases move (aerodynamics), and how high speed gases change (gas dynamics). Hydrodynamics, aerodynamics, and gas dynamics are all part of fluid dynamics, and each will be discussed in the next few pages.

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Last modified: Mon Jun 15 20:45:02 PDT 1998

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