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Technologies K–10 · Years 3–4

Balancing a ruler lever with washers

Physical World (NSW Science and Technology K–6, 2017); Design and Technologies: Knowledge and understanding, Technologies context: Engineering principles and systems (ACARA v9)

Practical, model not builtLow risk

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The idea

A lever balances when the number of equal loads times their distance from the pivot is the same on both sides, so a small load far from the pivot balances a big load close to it.

What you need

  • 1 flat 30 cm plastic ruler
  • 1 round pencil taped to the desk as the pivot
  • 10 identical metal washers (check on a scale that they have the same mass)
  • Recording table: washers left, distance left, washers right, distance right, balanced or not

How to do it

  1. Balance the empty ruler on the pencil and mark the balance point; measure all distances from it.
  2. Stack 2 washers on the left at 12 cm. Predict where a stack of 4 washers must go on the right to balance.
  3. Test the prediction by sliding the stack of 4 until the ruler balances. Record the distance.
  4. Repeat with 3 washers at 10 cm against 5 washers, then 1 washer at 12 cm against 3 washers, then 4 washers at 3 cm against 2 washers.
  5. Look for the rule that links washers and distance on both sides and write it in words.
  6. Use the rule to design a lever that lets 1 washer lift 4.

What you should see

The ruler balances when washers × distance is equal on both sides: 2 × 12 cm = 4 × 6 cm; 3 × 10 cm = 5 × 6 cm; 1 × 12 cm = 3 × 4 cm; 4 × 3 cm = 2 × 6 cm. The learner knows it worked when each predicted position balances the ruler to within the width of one washer; friction at the pencil pivot allows a small range of positions to look balanced.

What changes

What you change
the distance of the right-hand stack from the pivot
What you measure
whether the ruler balances
What you keep the same
  • identical washers
  • the same ruler and pivot
  • stacks centred on the measured mark

Common misconceptions

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

  • The heavier side always goes down (a light load far from the pivot can lift a heavy load close to it).
  • The pivot must be in the middle of the load (it can be anywhere; the distances decide the balance).
  • Doubling the distance makes no difference (doubling the distance doubles the turning effect).

Safety card

Low riskLearners carry it out

Hazards

  • Washers rolling onto the floor

Controls

  • Washers kept in a tray

Note

No chemicals or heat.

Curriculum references

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

  • Science and Technology K–6 Syllabus (2017), NESA. The syllabus taught in 2026; the 2024 syllabus replaces it from 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.ST2-9PW-STST2-2DP-T
  • Science and Technology K–6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.ST2-DDT-01
  • Australian Curriculum v9AC9TDE4K02

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. primaryconnections.org.au/v84-sequences/machine-makers
  4. www.teachengineering.org/lessons/view/cub_simp_machines_lesson01

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