1
Using a centimeter ruler
Discover Mathematics P2: Term 1, Lessons 41–50
GoalMeasure the length of an object in centimeters by starting at zero on the ruler.
New words
length · how long something iscentimeter (cm) · a small unit for measuring lengthruler · a tool with numbers for measuring lengthmeasure · find out how long something is
Explanation
We measure length with a ruler. Put the left end of the object exactly at the 0 mark. Keep the ruler still and straight. Look at the number where the other end of the object stops. That number tells how many centimeters long the object is. The picture shows a crayon that stops at 8, so it is 8 cm long. We write cm after the number.
Worked examples
A paper clip starts at 0 and stops at 3. It is 3 cm long.
Habiba’s pencil stops at 12. It is 12 cm long.
An eraser is 5 cm long. A ribbon is 9 cm long. The ribbon is 4 cm longer because 9 − 5 = 4.
Class activity
Measure at home. With a ruler, measure a spoon, a toothbrush, a book and your foot. Before you measure, guess the length. Write each guess and each real measurement in cm in your notebook. Which guess was the closest?
Practice
1
How long is the line?
6 cm.
2
How long is the ribbon?
11 cm.
3
One ribbon is 4 cm long and a second ribbon is 7 cm long. How much longer is the second one?
7 − 4 = 3. It is 3 cm longer.
4
Salma starts measuring her pencil at the 1 on the ruler and reads 13. Is 13 cm the right length?
No. We must start at 0. The pencil is 12 cm long.
5
Which unit is best for the length of a pencil: cm or kg?
cm.