# Basic Definitions and Circuit Laws

Chapter

## Abstract

It may be shown that all

*quantities*required in mechanics may be expressed in terms of three basic physical quantities. Although many sets of three quantities may be used as the basic one, the most natural, and the one most universally used is that of mass, length and time. In the SI (Système Internationale d’Unités) system of units the fundamental*units*associated with the basic physical quantities arc the metre, kilogram and second which are defined respectively in terms of the wavelength of the radiation of the orange line of krypton 86; the mass of a particular block of platinum located at Sèvres in France; and the period of oscillation of the caesium atom. All other mechanical quantities may be expressed in terms of the three basic quantities raised to various powers, which may he positive or negative, and which are termed the*dimensions*of the derived quantity. For example, force is defined in terms of the acceleration imparted to a given mass and hence the dimensional equations are
and the dimensions of force may be written as

$$\left[ {1,1, - 2} \right]$$

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## Copyright information

© G. Williams 1973