| WEIGHTS & MEASURES |
Two systems of weights and measures coexist in the United States
today: the U.S. Customary System and the International System of Units
(SI, after the initials Systeme International). The SI System, commonly
identified with the metric system, is actually a more complete and
coherent version of it. Throughout U.S. history, the Customary System
(inherited from, but now different from, the British Imperial System)
has been generally used.
The use of the SI System has slowly and steadily increased in the
United States, particularly in the scientific community; however, the
general public still uses, almost exclusively, the U.S. Customary
System. Because this Web site has a large international audience, all
calculations and example problems make exclusive use of SI units,
however the formulae provided will work in either system of units.
Whenever a basic constant is given, its U.S. Customary equivalent is
also given in parentheses.
In the SI System, the basic units are the units of length, mass,
and time, and are called respectively, the meter (m), the kilogram (kg),
and the second (s). The unit of force, called the Newton (N), is a
derived unit and is defined as the force that gives an acceleration of 1
m/s2 to a mass of 1 kg.
In the U.S. Customary System the base units are the units of
length, force, and time, and are called respectively, the foot (ft), the
pound (lb), and the second (s). The unit of mass, called the slug, is a
derived unit and is defined as the mass that receives an acceleration
of 1 ft/s2 when a force of 1 lb is applied to it.
When working in the U.S. Customary System, it is commonplace to
express "mass" in pounds; however, when doing so, it is necessary to
recognize that we are actually expressing "weight", which is the measure
of the gravitational force on a body. When used in this way, the weight
is that of a mass when it is subjected to an acceleration of one g. In the study of dynamics, where forces, masses, and accelerations are involved, it is important that we express the mass m in slugs of a body of which the weight W has been given in pounds. That is, m=W/g, where g is approximately equal to 32.174 ft/s2.
It is sometimes common practice for mathematical equations to be
expressed in such a way that the variable for mass is entered in the
unit of pounds. These equations include a term that converts mass from
pounds into its proper unit of slugs. When using these equations, one
must proceed with caution because they are only valid when using U.S.
Customary units. When using SI units, it is necessary to use a different
equation that does not contain the conversion factor. To avoid this
problem, all equations used in this Web site require that mass be
expressed in the unit of slugs in the U.S. Customary System, and
kilograms in the SI System.
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Penulis : Drs.Simon Arnold Julian Jacob
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