Units and De¢nitions of Material
A.1 UNIT SYSTEMS
The traditional unit system in the United States has been the Imperial system,
often referred to as the British system, although in United Kingdom the Imperial
system was replaced by the SI International System (Système Internationale,
French). In the United States, the engineering societies are in favor of adopting
SI, and most engineering publications and textbooks currently use SI units. Many
engineering companies are in transition from Imperial to SI units, so engineers
must be familiar with the two systems. For this reason, this text uses both
systems, although most of the example problems are presented in SI units.
The SI is based on three units: mass, length, and time. The unit of mass is
the kilogram (kg), that of length is the meter (m), and that for time is the second
(s). The unit of force is the Newton (N), which is defined by Newton’s second law
as the force required to accelerate 1 kg of mass at the rate of 1 m2=s.
Gravitational acceleration is g ¼ 9:81 m2=s, so the weight (force exerted by
gravity at the earth’s surface) of 1 kg mass is
F ¼ mg ¼ 1 9:81 ¼ 9:81 N
The unit of energy (or work) is the Joule (J), which is equivalent to N-m. The unit
of power, which is energy per unit of time, is the watt (W). The watt is equivalent
to J=s, or, in basic SI units, N-m=s.
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
Pressure or stress is force per unit area. The SI unit is the pascal (Pa), which
is equivalent to N=m2. This is a small unit, and prefixes such as kPa (103 Pa) and
MPa (106 Pa) are often used.
In SI units, very large or very small numbers are often needed in practical
problems, and the following prefixes serve to indicate multiplication of units by
various powers of 10:
m ðmicro-Þ ¼ 106
k ðkilo-Þ ¼ 103
m ðmilli-Þ ¼ 103 M ðmega-Þ ¼ 106
c ðcenti-Þ ¼ 102
G ðgiga-Þ ¼ 109
For example, the well-known Imperial unit of pressure is psi (lbf=in:
1 psi is ¼ 6895 N=m2 ðPaÞ ¼ 6:895 kPa:
A second example is t