## Length

The two basic mathematical ideas of measuring objects is the repetition of a unit of measurement and the assignment of a number that indicates how the unit compares with the object. For example, if a table is 5 feet long, the number 5 tells the number of times the unit (foot) repeats as it is compared to the length of the table.

Every measurement is approximate. The precision of measurement depends on the unit used for measuring an object. The smaller the unit used, the more precise the measurement.

While in real life only standard units are used for measuring objects, considering nonstandard units is helpful in developing children's understanding of the nature of measurement.

Objects can be measured by a variety of their attributes. The attributes considered at this grade level are length, weight, mass, capacity, and temperature. 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. In the metric system the units for measuring length are millimeter, centimeter, decimeter, meter, dekameter, hectometer, and kilometer.

Relationships among the metric units are based on the number ten.

1 kilometer = 10 hectometers
1 hectometer = 10 dekameters
1 dekameter = 10 meters
1 meter = 10 decimeters
1 decimeter = 10 centimeters
1 centimeter = 10 millimeters

Perimeter and Area
The perimeter of a polygon is the distance around the polygon, or the sum of the lengths of the sides. Since perimeter is a length, or distance, the unit of measure used may be inches, feet, centimeters, or meters. At this grade level, perimeter is introduced through a hands-on measurement activity.

Area is the amount of surface covered by a shape. The unit of measure for area is square units, such as square feet, square inches, or square centimeters. At this grade level, area is introduced by having children find how many square units are needed to completely cover a region.

Teaching Model 17.3: Inches and Feet