Choose a measurement unit
Measure length, mass and liquid volume using appropriate instruments and units.
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First decide what you are measuring
A desk has a length. A bag of rice has a mass. A bottle holds a volume of liquid. These are different quantities, so they need different units and instruments.
| Quantity | Useful unit | Instrument or comparison |
|---|---|---|
| Length of a room | metre, m | metre ruler or measuring tape |
| Mass of a person | kilogram, kg | suitable weighing scale |
| Mass of a small packet | gram, g | suitable weighing scale |
| Volume of water | litre, ℓ | marked measuring jug |
A centimetre is useful for a short pencil; a metre is useful for a room. One metre contains 100 centimetres. One kilogram contains 1000 grams. Choose a unit that makes the measurement easy to describe.
Keep the abbreviation with the number: 3 m, 200 g, 2 kg and 1 ℓ. The symbol ℓ is a letter used for litre, not the number 1. Abbreviations do not take an extra s for several units.
Check your understanding 1 Match the unit name and abbreviation
Choose the abbreviation for each unit.
Show answer
Metres use m, grams use g, kilograms use kg, and litres use ℓ. First choose a suitable unit for what you are measuring, then record its abbreviation correctly.
Read the scale, not the instrument’s shape
A scale is labelled 0 g, 100 g, 200 g, 300 g. If a pointer is exactly at 200 g, the mass is 200 g. What if it points to an unlabelled mark?
The 100 g gap is split into two equal gaps. Each smaller gap is 50 g.
Start at 100 g and move one small gap: 100 + 50 = 150. The pointer shows 150 g. Count the gaps, not just the printed numbers.
Check the value of the gaps before reading. A bigger-looking packet need not have greater mass: a large empty box can be lighter than a small full one.
Some weighing scales are curved. Follow the marks along the curve and count the equal gaps between labelled numbers. The needle shows a position; it does not change what one gap means.
Read both ends of a length
A strip starts at the 1 m mark on a measuring tape and ends at 4 m. Its length is the distance between those marks, not the number at the far end.
Check your understanding 2 Remove the part before the strip
How long is that strip?
Show answer
3 m, because 4-1 = 3. Moving the strip to start at zero would put its other end at 3 m.
Read the liquid surface
Place a measuring container on a flat surface and look at the scale at eye level. For water, read the bottom of the curved surface. Looking from above or below can put it beside the wrong mark.
Check your understanding 3 Find the value of one small gap
The container is labelled 0, 2, 4 and 6 litres. There is one extra mark between each pair of labelled marks. How much liquid does it show?
A measuring container with marks
Compare the bottom of the curved surface with the equally spaced scale marks.
View figure data
| Feature | Visible scale information |
|---|---|
| Unit | ℓ |
| Labelled marks | 0; 2; 4; 6 |
| Equal gaps between labels | 2 |
| Liquid surface | The bottom of the liquid surface aligns with small mark 1 after the labelled 2 mark. |
Show answer
3 ℓ. The gap from 2 ℓ to 4 ℓ contains two equal steps of 1 ℓ. The bottom of the curve lines up with the middle mark. Count the gaps and use the bottom of the curve, rather than its higher edges.
Compare using matching units
A 3 m ribbon is longer than a 2 m ribbon. A 500 g packet has greater mass than a 300 g packet. A jug with 2 ℓ of water contains more liquid than one with 1 ℓ.
For a fair volume comparison, use the marks on the jugs or pour into the same measuring jug. Water can stand higher in a narrow container while its volume stays the same.
To order several measurements, start with the smallest, then find the next smallest. Three ribbons measure 3 m, 1 m and 2 m. From shortest to longest, they go 1 m, 2 m, 3 m. All three use metres, so you can compare their numbers directly.
Check your understanding 4 Put three volumes in order
Three jugs hold 3 ℓ, 5 ℓ and 2 ℓ of water. Put them in order from least to most liquid. Say your order or write it down, including the unit.
Show answer
2 ℓ, 3 ℓ, 5 ℓ. Each reading is in litres. The jug’s height or width does not decide the order; the measured volume does.
Check your understanding 5 Read and compare masses
A scale shows 600 g for one packet and 350 g for another. How much greater is the first mass?
Show answer
250 g. The subtraction compares masses in the same unit; it does not find the mass of the empty scale.
Make the units match before comparing
Do not compare the numbers alone when the units differ. A 2 m ribbon is 200 cm long, so it is longer than a 150 cm ribbon. The smaller written number, 2, describes the greater length here because metres are bigger units.
Similarly, 1 kg is 1000 g. A 1 kg packet has greater mass than a 750 g packet, and the same mass as a 1000 g packet. Changing the unit changes the number used to describe the mass; it does not change the packet.
Check your understanding 6 Start with the greatest mass
Packet A has a mass of 1 kg, packet B has a mass of 350 g and packet C has a mass of 600 g. Name or point to the packets from heaviest to lightest. Use their measured masses, even if the packets look different sizes.
Show answer
A, C, B. First write 1 kg as 1000 g. The masses go 1000 g, 600 g, 350 g. Start with the greatest number for heaviest first; start with the least number for lightest first. Use each packet once.
Measure something real
Use a tape to measure a table’s length. Start at zero and keep the tape straight. Use a scale to weigh a safe small object. Read its unit before recording the result. With an adult, use a measuring jug on a flat surface to measure water at eye level.
Check your understanding 7 Choose and explain
You need the amount of water to fill a bottle. Would you measure metres, grams or litres? Which instrument helps? Why is the height of water in a different bottle not enough?
Show answer
Use litres and a measuring jug for liquid volume. Metres measure length and grams measure mass. Different bottle widths can give different water heights for the same volume. Read the jug scale rather than comparing height alone.
A written answer can check your interpretation of a reading. It cannot check how you placed or read a real instrument; compare that action with a person helping you.
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