Listed below are formal definitions of base units as provided by the Bureau International
des Poids et Mesures.Themeteris the length of the path traveled by light in a vacuum during a time interval of
1/299,792,458 of a second.
Thekilogramis the unit of mass; it is equal to the mass of the international prototype of the
kilogram.
Thesecondis the duration of 9,192,631,770 periods of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Theampereis that constant current which, if maintained in two straight parallel conductors
of infinite length, of negligible circular cross section, and placed 1 meter apart in a vacuum,
would produce between these conductors a force equal to 2 10
7
newton per
meter of length.
Thekelvin, a unit of thermodynamic temperature, is the fraction 1/273.16 of the thermody-
namic temperature of the triple point of water (a point at which ice, liquid water, and
water vapor coexist). The unit of Kelvin is related to the degree celsius, according to
KC273.16.
Themoleis the amount of substance of a system that contains as many elementary entities as
there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary
entities must be specified and may be atoms, molecules, ions, electrons, other particles, or
specified groups of such particles.
Thecandelais the luminous intensity, in a given direction, of a source that emits monochro-
matic radiation of frequency 540 10
12
hertz and that has a radiant intensity in that direc-
tion of 1/683 watt per steradian.You need not memorize the formal definitions of these units. From your everyday life expe-
riences you have a pretty good idea about some of them. For example, you know how short a
time period a second is, or how long a period a year is. However, you may need to develop a
“feel” for some of the other base units. For example, How long is a meter? How tall are you?
Under 2 meters or perhaps above 5 meters? Most adult people’s height is approximately between
1.6 meters and 2 meters. There are exceptions of course. What is your mass in kilograms? Devel-
oping a “feel” for units will make you a better engineer. For example, assume you are design-
ing and sizing a new type of hand-held tool, and based on your stress calculation, you arrive at
an average thickness of 1 meter. Having a “feel” for these units, you will be alarmed by the
value of the thickness and realize that somewhere in your calculations you must have made a
mistake. We will discuss in detail the role of the base dimensions and other derived units in the
upcoming chapters in this book.
The CGPM in 1960 adapted the first series of prefixes and symbols of decimal multiples
of SI units. Over the years, the list has been extended to include those listed in Table 6.2. SI is
the most common system of units used in the world.
The units for other physical quantities used in engineering can be derived from the
base units. For example, the unit for force is the newton. It is derived from Newton’s second law
of motion. One newton is defined as a magnitude of a force that when applied to 1 kilogram
of mass, will accelerate the mass at a rate of 1 meter per second squared ( m/s
2
). That is:
1N(1 kg)(1 m/s
2
).
Examples of commonly derived SI units used by engineers are shown in Table 6.3. The
physical quantities shown in Table 6.3. will be discussed in detail in the following chapters of134 Chapter 6 Fundamental Dimensions and Units
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