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From our everyday experience, we have become familiar with the fact that matter occurs in three different forms-solid, liquid, and gas. Under ordinary conditions stone, iron, copper, and chalk, for example, are solids; water, oil, and mercury are liquids; air, hydrogen, and carbon dioxide are gases. Each one of these forms is called a phase. At times it is difficult to distinguish clearly between the solid and the liquid phases, as in a material such as tar which flows under the action of a force at ordinary temperatures. Metals at high temperatures flow or "creep" under the action of a force. Even where the different phases are clearly recognizable, materials undergo a phase change under different conditions of temperature and pressure. For the present we shall confine our discussion to the application of the principles of mechanics to bodies which remain in the same phase.
Liquids and gases are sometimes grouped together as fluids because they flow very readily under the application of an external force, while solids do not. A solid has a definite size and a definite shape, and these. change only slightly under the application of external forces. For this reason it is possible to study the statics of solids by characterizing them as rigid bodies. Liquids, on the other hand, possess a definite size or volume but change their shape very readily. Liquids at rest generally take the shape of the containing vessel. If the containing vessel has a volume greater than that of the liquid put into it, there will be a free surface at the top of the liquid. A gas differs from a liquid in that a gas has neither size nor shape. A quantity of gas placed in a container will completely fill that container. There is no free surface. The volume of the gas is the volume of the container.