Fundamental dimensions and units

There are four fundamental dimensions in terms of which the dimensions of all other physical quantities may be expressed. They are mass [M], length [L], time [T] and temperature [ff J A consistent set of units is formed by specifying a unit of particular value for each of these dimensions. In aeronautical engineering the accepted units are respectively the kilogram, the metre, the second and the Kelvin or degree Celsius (see below). These are identical with the units of the same names in common use, and are defined by international agreement.

It is convenient and conventional to represent the names of these units by abbreviations:

kg for kilogram m for metre s for second °С for degree Celsius К for Kelvin

The degree Celsius is one one-hundredth part of the temperature rise involved when pure water at freezing temperature is heated to boiling temperature at standard pressure. In the Celsius scale, pure water at standard pressure freezes at 0 °С and boils at 100 °С.

The unit Kelvin (K) is identical in size with the degree Celsius (°С), but the Kelvin scale of temperature is measured from the absolute zero of temperature, which is approximately —273 °С. Thus a temperature in К is equal to the temperature in °С plus 273 (approximately).