Mathematics K–6 · Years 5–6

Which scales: choosing units and instruments for mass

Measurement and space

PracticalLow risk

The idea

Different scales read to different steps, so the unit and the instrument are chosen to suit the object, and grams, kilograms and tonnes convert by thousands.

What you need

digital kitchen scales reading to 1 g (up to 5 kg)
bathroom scales reading to 0.1 kg
an equal-arm balance with a set of gram masses (10 g, 20 g, 50 g, 100 g, 200 g, 500 g)
objects: a paperclip, an apple, a 1 L bottle of water, a school bag and a stack of ten textbooks

How to do it

For each object, decide before weighing which instrument and unit suit it, and write the reason (too light, too heavy, needs grams).
Weigh the paperclip on all three instruments and record what each shows.
Weigh the apple and the water bottle on the kitchen scales in grams and write each in kilograms as a decimal.
Weigh the school bag and the textbook stack on the bathroom scales in kilograms and write each in grams.
Work out how many 1 L bottles of water would have a mass of one tonne, and how many school bags of the mass you measured.

What you should see

A paperclip of about a gram reads 0.0 kg on the bathroom scales, which cannot resolve it, and at most a gram or two on the kitchen scales, at the limit of their 1 g step; on the balance it is lighter than every mass in a set that starts at 10 g. No instrument here weighs a single paperclip well, which is why the instrument is chosen to suit the object. A 1 L bottle of water at 20 degrees Celsius holds 998 g of water, so with the bottle it reads a little over 1 kg. A school bag of 4.3 kg is 4300 g. One tonne is 1000 kg, so about 1000 litre bottles of water or about 233 school bags of 4.3 kg make a tonne.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Any scales will do for any object.
Bigger objects always have more mass than smaller ones.

Safety card

Low riskLearners carry it out

Hazards

lifting a full school bag or a stack of textbooks onto scales

Controls

lift with a straight back, or two learners lift together

Note

No chemicals or heat are used, so the NSW Department of Education Chemical Safety in Schools package is not needed; ordinary classroom supervision under the school's usual risk management.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Mathematics K-10 Syllabus (2022), NSW Education Standards AuthorityMA3-NSM-01 MAO-WM-01
Australian Curriculum v9AC9M5M01AC9M6M01

Sources

The pages the author read to write this activity.

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