Page 18 - Aerodynamics for Engineering Students

Basic concepts and definitions

1.1 Units and dimensions

A study in any science must include measurement and calculation, which presupposes
an agreed system of units in terms of which quantities can be measured and expressed.
There is one system that has come to be accepted for most branches of science and
engineering, and for aerodynamics in particular, in most parts of the world. That
system is the Systeme International d’Unitks, commonly abbreviated to SI units, and it
is used throughout this book, except in a very few places as specially noted.
It is essential to distinguish between the terms ‘dimension’ and ‘unit’. For example,
the dimension ‘length’ expresses the qualitative concept of linear displacement,
or distance between two points, as an abstract idea, without reference to actual
quantitative measurement. The term ‘unit’ indicates a specified amount of the quantity.
Thus a metre is a unit of length, being an actual ‘amount’ of linear displacement, and