Concepts
1 BasicFunctions
Copyright © Cengage Learning. All rights reserved.
Unit 1B
Review of Operations with
Fractions
Copyright © Cengage Learning. All rights reserved.
1.9
The U.S. System of
Weights and Measures
Copyright © Cengage Learning. All rights reserved.
The U.S. System of Weights and Measures
Measurement is the comparison of an observed quantity
with a standard unit quantity.
A standard unit that is constant, accurate, and accepted by
all is needed for technical measurements.
The U.S. system of weights and measures, which is
derived from and sometimes called the English system, is a
combination of makeshift units of Anglo-Saxon, Roman,
and French-Norman weights and measures.
4
The U.S. System of Weights and Measures
The metric system, which is now used by international
industry and business, all major U.S. industries, and most
federal agencies.
The U.S. system requires us to understand and be able to
use fractions in everyday life.
One advantage of the metric system is that the importance
of fractional computations is greatly reduced.
5
Example 1
Change 5 ft 9 in. to inches.
1 ft = 12 in.,
so
5 ft = 5  12 in.
= 60 in.
5 ft 9 in. = 60 in. + 9 in.
= 69 in.
6
The U.S. System of Weights and Measures
To change from one unit or set of units to another, we use
what is commonly called a conversion factor.
We know that we can multiply any number or quantity by
1 (one) without changing its value.
We also know that any fraction whose numerator and
denominator are the same is equal to 1.
7
The U.S. System of Weights and Measures
For example,
,
Also, since 12 in. = 1 ft,
, and
.
, and likewise,
,
because the numerator equals the denominator.
We call such names for 1 conversion factors (or unit
conversion factors).
8
The U.S. System of Weights and Measures
Choosing Conversion Factors
The correct choice for a given conversion factor is the one
in which the old units are in the numerator of the original
expression and in the denominator of the conversion factor,
or the old units are in the denominator of the original
expression and in the numerator of the conversion factor.
That is, set up the conversion factor so that the old units
cancel each other.
The use of a conversion factor is especially helpful for units
with which you are unfamiliar, such as rods, chains, or
fathoms.
9