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

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 round piece of wood is placed in a square container, its shape does not change to match the container. In dynamics, this is called a non-deformable body (no automatic shape changing). 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 square container into a round one, will not retain its square 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. The liquid will not, however, expand to fill a larger space. It cannot be made smaller by squeezing or pulling. The molecules do not move closer or further apart. Liquid is incompressible.

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. A gas can be expanded or compressed. Another example is compressed air in a cylinder used by a diver. Another name for liquids and gases is "fluid". A fluid is deformable.

The study of dynamics, then, can be split 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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