72
Units of length
Textbook: Part 3, lesson 72, pp. 13–15
GoalUnits of length mm, cm, dm, m and how they are related: 1 cm = 10 mm, 1 dm = 10 cm, 1 m = 10 dm = 100 cm = 1 000 mm.
New words
millimetre · millimetr (mm)centimetre · santimetr (sm)decimetre · detsimetr (dm)metre · metr (m)
Explanation
The length of a segment is the number of unit segments that fit in it. Today the main unit all over the world is the metre. Cut a metre into 10 equal parts and each is a decimetre; cut a decimetre into 10 and each is a centimetre; cut a centimetre into 10 and each is a millimetre. So 1 m = 100 cm = 1 000 mm. Long ago people used finger width (inch), forearm (cubit) and foot as units.
Worked examples
3 dm 6 cm = 30 cm + 6 cm = 36 cm; 5 m = 500 cm.
7 cm 3 mm = 70 mm + 3 mm = 73 mm; 2 dm 5 cm = 250 mm.
Class activity
“Ruler hunt”: children measure the length of their desk with a ruler, write the result in cm, dm and mm, and compare with a neighbour.
Practice
1
4 m 5 dm = ? dm
45
2
6 cm 8 mm = ? mm
68
3
2 m 3 cm = ? cm
203
4
A room is 6 m 40 cm long and 4 m 20 cm wide. What is its perimeter in centimetres?
2120
5
One ruler is 30 cm long. Five rulers are placed end to end. How many decimetres long is the line?
15