The idea, step by step
Understand the idea
A measurement combines a number with a unit. A room length and a wire diameter are both lengths, but metres suit the first and millimetres often suit the second. Identify the quantity before choosing an instrument: a scale measures mass, not capacity.
An instrument needs sufficient range and suitably small divisions. A tape can measure a long fence; a calliper can resolve small component diameters. More display digits do not automatically establish better accuracy.
Choose and apply a method
Align a ruler with the object, view a scale straight on and check its zero. If an edge is damaged, read the positions of both ends and subtract. Record the unit immediately rather than trying to remember it later.
Explain the instrument choice in relation to the task. A shelf-fit check may need millimetre resolution, whereas estimating a running-track distance does not need fractions of a millimetre.
A common trap
Choosing a device only because its display has many digits.
Instead: Match quantity, range and resolution, and check calibration when accuracy matters.
See the thinking
Match instruments to quantities
Choose instruments and units for a 17 m fence and a 250 g seed packet.
- The fence is a long length requiring sufficient range.
- The packet requires a mass instrument with gram resolution.
Use position readings
A strip spans ruler readings 1.8 cm and 9.6 cm. Find its length.
- Subtract positions: .
- Retain centimetres as the length unit.
Check your understanding
Would a bathroom scale reliably measure 3 g of spice?
Show answer
Usually no.
Its normal smallest change is much larger than 1 g; choose a suitable kitchen or laboratory scale.
Your turn
6 questions · 12 marks · Work at your own pace.
Read the questions online
- Choose a sensible unit for a swimming-pool length and explain it.2 marks
- Choose an instrument to measure 75 mL of liquid to the nearest millilitre.2 marks
- A window width must be checked to the nearest millimetre. Is a tape marked only every centimetre sufficient?2 marks
- A rod spans ruler readings 2.4 cm and 15.9 cm. Find its length.2 marks
- A carton is labelled 2 L. Is this mass or capacity? Name a suitable measuring instrument.2 marks
- Explain why reporting a width as 42.000 mm from a ruler with 1 mm divisions is misleading.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Choose a sensible unit for a swimming-pool length and explain it.2 marks
A pool is normally tens of metres long; millimetres would be unnecessarily small.
Metres. - Choose an instrument to measure 75 mL of liquid to the nearest millilitre.2 marks
It measures liquid volume and resolves the required 1 mL.
A suitable graduated measuring cylinder with 1 mL divisions. - A window width must be checked to the nearest millimetre. Is a tape marked only every centimetre sufficient?2 marks
A centimetre is 10 mm, so the marks do not directly resolve each millimetre.
No. - A rod spans ruler readings 2.4 cm and 15.9 cm. Find its length.2 marks
cm.
13.5 cm. - A carton is labelled 2 L. Is this mass or capacity? Name a suitable measuring instrument.2 marks
Litres measure capacity, not mass.
Capacity; a graduated liquid-volume measure. - Explain why reporting a width as 42.000 mm from a ruler with 1 mm divisions is misleading.2 marks
Choose a recorded precision consistent with the scale and reading method.
The decimal places imply more resolution than the ruler supplies.
Curriculum reference & lesson scope
SACE Board of South Australia
Stage 1 General Mathematics measurement skills and applications; original explanatory lessons and papers.
Official curriculum source ↗