Measuring and Comparing Length
Once the children have had experience measuring with nonstandard units, they are ready to make the transition to measuring in inches and centimeters.
Materials: an inch ruler and a centimeter ruler for each child or pair of children; a plastic bag containing the following items for each child or pair of children: an unsharpened pencil, a new crayon, a felt-tipped marker, an index card, a paintbrush, a new piece of chalk, and a craft stick; a copy of the Inches and Centimeters worksheet for each child; several clear plastic inch and centimeter rulers for use on the overhead
Preparation: Collect the items listed above and assemble the bags. Make a copy of the worksheet for each child.
Prerequisite Skills and Concepts: Children should know how to measure using nonstandard units. They should understand how to align the edge of the nonstandard unit and the object being measured to get an accurate measurement.
- Say: Last time we learned how to measure with paper clips. Do you remember that long ago people used to measure things with their hands and feet? Let's measure the length of a section of the blackboard using our hands.
Show the children how to measure a section of the board by putting your hands one in next to the other and counting. Record your measurement in a chart on the board. Then call up 5 volunteers to measure the same section. Record their measurements.
- Ask: Why did we get different measurements? We all used our hands, and we all measured the same section of the board.
Lead the children to see that the children in the class have hands of different sizes; therefore, they had different measurements.
- Say: So the size of the unit you are measuring with affects the measurement.
- Ask: Let's pretend you need a board to build something. With your hands you measure how long the board must be. When you got to the store, you tell the clerk how long a board you need. He measures the length using his hands. Would that be all right?
Lead the children to see that since the clerk's hands and your hands are probably different sizes, you might get a board that is the wrong length.
- Say: This is exactly what people realized long ago. They decided that in order to make measurement useful they needed a unit of measure that was always the same size, no matter who was measuring. Today we are going to learn how to measure with two different unitsinches and centimeters. (Place a clear inch ruler on the overhead.) This is an inch ruler. Raise your hand if you have seen an inch ruler before.
Most of the children will probably raise their hands.
- Say: The space between each of these marks is one inch long. (Frame two tick marks with your fingers.) Inches are always the same size. No matter which ruler you use, an inch is always the same length. So it not like using paper clips, hands, or feet, where the size of the unit can change.
Place several clear rulers on the overhead, one next to the other. Show children that an inch is always the same length.
- Say: To measure with an inch ruler, line up the end of the ruler with whatever you want to measure, just as you lined up the end of the paper clip with whatever you wanted to measure.
On the overhead, demonstrate how to measure a felt-tipped marker. Show children how to measure to the nearest inch by deciding which number is closest to the end of the object. Pass out the plastic bags, rulers, and Inches and Centimeters worksheet to each child or pair of children. Each child should get a worksheet even if he or she is working in pairs.
- Say: Now we are going to practice measuring with inch rulers. Take the pencil from the bag that I just gave you. Line up the end of the pencil with the end of the ruler, like this.
Place a pencil on the overhead and demonstrate how to measure the length of the pencil. Have children follow along with their pencil and ruler and then record the length in inches on their worksheet. To prevent them from writing in the wrong column, you may wish to have children fold their worksheets so they cannot see the centimeter column. You may need to model measuring one more item on the overhead and then allow the children time to measure each item in their bag. Have them record their results. When they are done, compare results. All results should be the same. If any are different, measure the item on the overhead.
- Say: Rulers can be used not only to measure items, but also to help you draw lines that are a certain length. This is how to draw a line that is 1-inch long.
Model how to use the ruler to draw a 1-inch line for the children. Have each child draw and label his or her own 1-inch line on the back of the worksheet. Remind children to start drawing at the left end of the ruler and stop at the tick mark that indicates the number of inches they want. Repeat modeling, drawing and labeling lines up to six inches. Collect the inch rulers and hand out the centimeter rulers.
- Say: An inch is a unit of measurement used in the United States. There is another unit, called a centimeter, that is used in most of the rest of the world. Even though a centimeter is smaller than an inch, just like an inch, a centimeter is always the same length.
Line up several centimeter rulers on the overhead to confirm that a centimeter is always the same length.
- Say: The space between each of these marks is one centimeter long. (Frame two tick marks with your fingers.) Centimeters are always the same size. No matter which centimeter ruler you use, a centimeter is always this length.
Model using a centimeter ruler to measure a marker on the overhead. Then have students re-measure all the items in their bags in centimeters. Compare and check results as with inches. Then demonstrate how to draw lines of specific lengths with a centimeter ruler.
Wrap-Up and Assessment Hints
Have children run ruler races. Give two children a number cube (1-6), an inch or centimeter ruler, two pieces of paper, and several markers or crayons. Have children take turns rolling the cube to determine the length of the line they should draw (e.g., 1cm, 2cm, 3cm, and so forth). Then have them draw a line that is that length. Each time they roll, have the children use a different color crayon to add that many inches or centimeters to their line. The first child to span the page wins. Children should work lengthwise across the paper if they are racing in inches and "widthwise" across the paper if they are racing in centimeters.