UNIT 2 MODULE 4

GEOMETRY: UNITS OF GEOMETRIC MEASURE

LINEAR MEASURE

In geometry, linear measure is the measure of distance. For instance, lengths, heights, and widths of geometric figures are distances, as are the radius, diameter and circumference of a circle. The perimeter of a figure is another example of distance.

Distance is measured in linear units, such as inches, feet, yards, miles, meters, centimeters, millimeters and kilometers. There are many other units of linear measure, but those listed above are the most common.

Converting Linear Units

In the United States, any educated person should be aware of the following relationships among basic units of linear measure in the English system (inches, feet, yards, miles), and among the basic units of linear measure in the metric system (meters, centimeters, millimeters, kilometers).

1 foot = 12 inches

1 yard = 3 feet

1 yard = 36 inches

1 mile = 5280 feet

1 meter = 1000 millimeters

1 meter = 100 centimeters

1 kilometer =1000 meters

EXAMPLE 2.4.1

1. How many feet are are 13 yards?

2. How many inches are in 4 yards?

3. How many meters are in 3 kilometers?

4. How many feet are in 120 inches?

5. How many meters are in 820 centimeters?

6. How many miles are in 20,000 feet?

EXAMPLE 2.4.1 SOLUTIONS

Notice that in the list of conversions given above, the units of measure in the left-hand column are larger than the corresponding units in the right-hand column. We convert from larger units to smaller units by multiplying (for instance, to convert from yards to inches we multiply by 36); we convert from smaller units to larger units by dividing (for instance, to convert from centimeters to meters we divide by 100).

1. To convert 13 yards to feet we multiply by 3.

(13 yards)(3 feet per yard) = 39 feet

2. To convert 4 yards to inches we multiply by 36.

(4 yards)(36 inches per yard) = 144 inches

3. To convert 3 kilometers to meters we multiply by 1000.

(3 km)(1000 m per km)= 3000 meters

4. To convert 120 inches to feet we divide by 12.

120/12 = 10 feet

5. To convert 820 centimeters to meters we divide by 100.

820/110 = 8.2 meters

6. To convert 20000 feet to miles we divide by 5280.

20000/5280 = 3.88 miles (we have rounded to two decimal places)

SQUARE MEASURE and CUBIC MEASURE

Square units (such as square inches or square centimeters) are used to describe the area of a two-dimensional figure. Area is the amount of 2-dimensional space covered by an object, for instance.

To understand the difference between linear measure and square measure, you must realize (for example) that one square inch is the area of a square that is 1 inch wide and 1 inch high.

Cubic units (such as cubic meters or cubic yards) are used to describe the volume of a three-dimensional figure. Volume is the amount of 3-dimensional space occupied by a solid object, for instance, or the amount of fluid that can be contained in a hollow vessel.

To understand the difference between linear measure and cubic measure, you must realize (for example) that cubic inch is the volume of a cube that is 1 inch long, 1 inch wide and 1 inch high.

Conversions involving square measure or cubic measure

EXAMPLE 2.4.2

How many square inches are in 2 square yards?

EXAMPLE 2.4.2 SOLUTION

We might expect that to convert square yards to square inches we multiply by 36. This is not correct, however. We must understand that the meaning of "squar" is both geometric and algebraic: "1 square yard" means "1 yard x 1 yard"

1 square yard = 1 yard x 1 yard

1 square yard = (36 inches)(36 inches) = 1296 square inches

2 square yards = 2(1296) = 2592 square inches

= 2592 square inches

EXAMPLE 2.4.3

How many cubic feet are in 10 cubic yards?

EXAMPLE 2.4.2 SOLUTION

We might expect that to convert cubic yards to cubic feet we multiply by 3. This is not correct, however. We must understand the meaning of "cubic," which is both geometric and algebraic:

"1 cubic yard" means " 1 yard x 1 yard x 1 yard"

1 cubic yard = (1 yard)(1 yard)(1 yard)

1 cubic yard = (3 feet)(3 feet)(3 feet) = 27 cubic feet

10 cubic yards = 10(27) = 270 cubic feet

= 270 cubic feet

For additional practice on problems like these try The Big Unit-Izer.




Download practice exercises (PDF file)