In this chapter we consider the concepts of dimensions and units and the various systems in use for describing these quantities. In particular, the distinction between scientific and engineering systems of dimensions is explained, and the various metric and English units used in each system are discussed. It is important that the engineer be familiar with these systems, as they are all in common use in various fields of engineering and will continue to be for the indefinite future. It is common to encounter a variety of units in different systems during the analysis of a given problem, and the engineer must be adept at dealing with all of them.

The concept of ''conservation of dimensions'' will then be applied to the dimensional analysis and scale-up of engineering systems. It will be shown how these principles are used in the design and interpretation of laboratory experiments on ''model'' systems to predict the behavior of large-scale (''field'') systems (this is also known as similitude). These concepts are presented early on, because we shall make frequent use of them in describing the results of both theoretical and experimental analyses of engineering systems in a form that is the most concise, general, and useful. These methods can also provide guidance in choosing the best approach to take in the solution of many complex problems.

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