7.2 Measurement of Length 165
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Early humans may have used finger length, arm span, stride length (step length), a stick, rope,
chains, and so on to measure the size or displacement of an object. Today, depending on how
accurate the measurement needs to be and the size of the object being measured, we use other
measuring devices, such as a ruler, a yardstick, and a steel tape. All of us have used a ruler or tape
measure to measure a distance or the size of an object. These devices are based on internationally
defined and accepted units such as millimeters, centimeters, or meters or inches, feet, or yards. For
more accurate measurements of small objects, we have developed measurement tools such as the
micrometer or the Vernier caliper, which allow us to measure dimensions within 1/1000 of an
inch. In fact, machinists use micrometers and Vernier calipers every day.
On a larger scale, you have seen the milepost markers along interstate highways. Some
people actually use the mileposts to check the accuracy of their car’s odometer. By measuring
the time between two markers, you can also check the accuracy of the car’s speedometer. In the
last few decades, electronic distance measuring instruments (EDMI) have been developed that
allow us to measure distances from a few feet to many miles with reasonable accuracy. These
electronic distance measuring devices are used quite commonly for surveying purposes in civil
engineering applications. The instrument sends out a light beam that is reflected by a system
of reflectors located at the unknown distance. The instrument and the reflector system are sit-
uated such that the reflected light beam is intercepted by the instrument. The instrument then
interprets the information to determine the distance between the instrument and the reflector.
The Global Positioning System (GPS) is another example of recent advances in locating objects
on the surface of the earth with good accuracy. As of the year 2006, radio signals were sent from
approximately 30 artificial satellites orbiting the earth. Tracking stations are located around the
world to receive and interpret the signals sent from the satellites. Although originally the GPSA Vernier caliper and a
micrometer.
Source:Saeed MoaveniCopyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀