Measurements and units

Learning objectives

• Choose an appropriate measuring instrument.
• Convert common SI prefixes.
• Separate a measurement from its uncertainty.

The big idea

A physical quantity combines a number and a unit. Length, time and mass are measured in metres, seconds and kilograms. A prefix changes the scale: a kilometre is one thousand metres.

Choose an instrument with suitable range and resolution. Repeat measurements when possible and report only precision that your instrument supports; extra decimal places do not make a result more accurate.

1. A measurement needs a meaning

Saying a desk is 120 long leaves the listener guessing. Saying its length is 120 cm identifies both the quantity and its unit. Choose the part you are measuring before placing the instrument: width, height and length are different descriptions of the same object.

2. Read the scale, not your expectation

Put a ruler’s zero beside the starting edge and look straight at the scale to reduce parallax. If the zero edge is damaged, start from another mark and subtract the two readings. The smallest scale division limits how finely you can reasonably report a result.

3. Convert without changing the object

A 120 cm desk is also 1.20 m long. Divide centimetres by 100 to obtain metres; the desk has not shrunk. Repeating a measurement can reveal inconsistent placement, but cannot fix a ruler that is systematically miscalibrated. Record the instrument and unit alongside your number.

A method you can reuse

1. Identify exactly which length or time you need.
2. Choose a suitable range and scale.
3. Read and repeat without changing the method.
4. Convert units, then report sensible precision.

Equations and relationships

Investigate and explain

Measure a book three times with a ruler. Record the scale division and compare the repeated readings.

Use the data tools to practise recording measurements and inspecting scatter. They do not reproduce ruler parallax or calibration; also measure a real everyday object.

Open the interactive investigation

Check your understanding