Every object on earth or in space can be classified as a solid, a liquid, or a gas. The science of "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 that line up in an even pattern and give the object a specific shape. A block of wood and a crystal of salt are solids. A solid has a fixed shape. For example, when 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 (the shape does not change). No matter how hard a solid is squeezed or pulled, its molecules do not move closer together or further apart. The object may break, but the molecules don't move. This is an "incompressible" object.
The molecules in a liquid, however, are not so well-organized. When poured from a square container into a round one, a liquid will not retain its square shape but will take the shape of the round container. It is deformable. The liquid will not, however, expand to fill a larger space. The liquid cannot be made larger or smaller by squeezing or pulling. The molecules do not move closer or further apart. Like the solids, a liquid is incompressible.
Gases, like air, have even less-organized molecules. Gases take the shape of their containers, and also expand or contract to fill the container. When a person breathes air in, 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.
Many people use the word "fluid" interchangeably with the word "liquid"; but scientifically this is not correct. "Fluid" refers to a liquid or a gas. A fluid is deformable.
The study of dynamics can be split into four areas: the dynamics of solids, hydrodynamics -- or how liquids and other incompressible fluids behave, aerodynamics - how air and other gases move, and gas dynamics how high speed gases change. Remember, a fluid can be a liquid or a gas. So, "hydrodynamics", "aerodynamics", and "gas dynamics" are all part of a larger area called "fluid dynamics", and each will be discussed in the next few pages.