## Length and Weight

The purpose of measuring an object is to find out how many times a measurement unit fits along the length of it. For example, to say that one side of a table measures 5 units is to say that the unit of measurement used fits 5 times along the side of the table.

Units of measure used can be standard or nonstandard. In ancient times, it was common practice to use the length of an arm as a unit for measuring length. The disadvantage of using this nonstandard unit is that different people have different length arms, which would result in different measurements for the same object. Using a standard unit of length is expected to give the same measurement no matter who does the measuring.

An important thing to remember is that every measurement is approximate. How precise a measurement is depends on the unit used for measuring an object. The smaller the unit, the more precise is the measurement made using that unit.

At this grade level, the length, (height), weight, mass, and capacity of objects are measured.

There are two basic systems of measurement used in the United States, customary and metric. The units of length in the customary system are inch, foot, yard, and mile. The units of weight in the customary system are ounce, pound, and ton.

In the metric system the units for measuring length are millimeter, centimeter, decimeter, meter, decameter, hectometer, and kilometer. The most frequently used metric units of length are the centimeter, meter, and kilometer. Unlike the customary system, there are very regular relationships among the metric units based on the number ten.

It is important to distinguish between the concepts of weight and mass. Weight varies depending upon the gravitational pull on an object. For example, because gravity on Earth differs from gravity on the moon, an object that weighs 12 pounds on Earth will weigh about 2 pounds on the moon. The mass of an object is its amount of matter. Whether on Earth or on the moon, the mass of an object would not change. The most commonly used metric units of mass are gram and kilogram.

Teaching Model 17.8: Use Logical Reasoning