BASIC METRIC MEASURES. Under the United States Customary
System of measurement, the inch, foot, or yard is used to measure
length, the pound is used to measure weight, and the gallon is used to
SI (or metric)
measure volume. In the SI or metric system, you would use meters for
length, grams for mass, and liters for volume.
When using the metric system:
Length is expressed in
Mass is expressed in
Liquid capacity is expressed in
NOTE: In the remainder of this lesson, the term "metric" will be used to
denote the SI system of measures.
Notice that when the U.S. system was discussed in Frame 4-4, the term
"weight" was used; but when the metric system was discussed, the term
"mass" was used. "Weight" measures gravity's attraction to a given
object (its "heaviness"). Mass is a measure of an object's resistance to
acceleration (its inertia). In other words, mass is a measure of how
much matter is in the object while weight measures the force exerted by
the object. For our purposes, we can say that weight and mass are the
same. An object with a mass of 40 kilograms (40,000 grams), for
example, will weight the same anywhere on the surface of the earth
since the earth's gravity exerts the same pull. This works as long as you
are dealing with the earth's gravity, but what happens if you are not? An
object with a mass of 40 kilograms weighs about 88 pounds on earth.
On the moon, the same object would weight about 15 pounds since the
moon's gravitational pull is only one-sixth that of the earth's gravity. The
object's mass, however, would remain unchanged (40 kilograms), but it
would feel as heavy as a 6.7 kilogram weight on earth. In orbit around
the earth, the object would be weightless (zero pounds), but still retain
its mass (inertia) of 40 kilograms.
NOTE: In the U.S. system, the unit used to measure mass is the slug
(about 14,594 grams).
In scientific matters, it is usually easier to speak of an object's
rather than its weight since its
does not change. (Einstein's
theories of relativity are not considered in this subcourse.)
NOTE: For the remainder of this lesson, there will be no distinction
between "weight" and "mass."