Week 5 · Length, mass and liquid volume · 3 lessons
1
Choosing the right unit of length
Discover Mathematics P4: Term 1, Unit 3, Lessons 1–4
GoalChoose the best metric unit to measure a length and change between km, m, cm and mm.
New words
kilometre (km) · a unit for long distances; 1 km = 1,000 mmetre (m) · a unit for rooms and roads; 1 m = 100 cmcentimetre (cm) · a unit for small objects; 1 cm = 10 mmmillimetre (mm) · a very small unit, the width of a thin line
Explanation
We choose a unit that fits the size of what we measure. A pencil is about 15 cm long. The door of a classroom is about 2 m high. The thickness of a coin is about 2 mm. The distance between two cities is measured in km. These units are linked by tens, hundreds and thousands: 1 km = 1,000 m, 1 m = 100 cm and 1 cm = 10 mm. To change a bigger unit to a smaller unit, multiply. To change a smaller unit to a bigger one, divide. For example 3 m = 3 × 100 = 300 cm. A length can also have mixed units: 2 km 350 m means 2,000 m + 350 m = 2,350 m. The smaller the unit is, the bigger the number of units we need.
Unit
Equals
Good for
1 km
1,000 m
roads, cities
1 m
100 cm
rooms, doors
1 cm
10 mm
pencils, books
1 mm
–
thin things
Worked examples
Object
Best unit
football pitch
m
notebook page
mm
road between cities
km
width of a book
cm
Which unit measures these? The length of a football pitch: metres. The thickness of a notebook page: millimetres. The road from Cairo to Alexandria: kilometres. The width of a book: centimetres.
From
To
Work
3 m
300 cm
3 × 100
450 cm
4 m 50 cm
450 ÷ 100
2 km 350 m
2,350 m
2 × 1,000 + 350
Change 3 m to cm: 3 × 100 = 300 cm. Change 450 cm to m and cm: 450 ÷ 100 = 4 remainder 50, so 4 m 50 cm. Change 2 km 350 m to metres: 2 × 1,000 + 350 = 2,350 m.
Omar walks 1,800 m to school. In km and m this is 1 km 800 m. Farida walks 2 km 100 m. Farida walks 2,100 − 1,800 = 300 m more than Omar.
Class activity
Measure and choose. Find five things at home: a spoon, a door, a pencil, a corridor and a paper clip. Decide the best unit for each (mm, cm or m), measure with a ruler or a tape, and write the length. Which object was longest in cm? Convert it into mm.
Practice
1
Which unit is best for the length of a bus: cm, m or km?
Metres (m): a bus is about 10 m long.
2
Change 7 m to cm.
7 × 100 = 700 cm.
3
Change 5 km 40 m to metres.
5 × 1,000 + 40 = 5,040 m.
4
Change 2,600 m to km and m.
2 km 600 m.
5
A ribbon is 3 cm 6 mm long. How many millimetres is that?
