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Science 7–10 · Year 7

Levers: balancing a metre rule to find the law of moments

Physical sciences — Forces (NSW Stage 4 focus area)

Practical, model not builtLow risk

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The idea

A lever balances when force times distance on one side of the pivot equals force times distance on the other, so a small force far from the pivot can balance a large force close to it.

What you need

  • wooden metre rule, 1
  • triangular pivot (knife-edge or a prism of wood), 1
  • slotted masses 50 g to 200 g with hangers, 2 sets
  • thread loops to hang masses, 4
  • 500 g slotted mass on a hanger, 1 (extension)
  • 0 to 2.5 N newton meter, 1 (extension)

How to do it

  1. Balance the empty rule on the pivot and note the balance point (close to 50.0 cm). All distances are measured from this point.
  2. Hang 100 g at 40.0 cm from the pivot on the left. Find by trial the distance on the right at which 200 g balances the rule; record it.
  3. Keep 100 g at 40.0 cm on the left and balance it in turn with 100 g and 150 g on the right, recording each balancing distance.
  4. Convert each mass to force (F = m g, g = 9.8 N/kg) and calculate force x distance for each side in newton-metres.
  5. Compare left and right products for every trial and state the rule they obey.
  6. Extension: hang 500 g at 10 cm and pull down with the newton meter at 40 cm on the other side until the rule is level; read the force needed.

What you should see

200 g balances 100 g at 40.0 cm when it hangs at 20.0 cm; 100 g needs 40.0 cm and 150 g needs 26.7 cm, all within the 50 cm available on each side of the pivot. Each side's product is the same: 0.100 kg x 9.8 N/kg x 0.400 m = 0.392 N m against 0.200 kg x 9.8 N/kg x 0.200 m = 0.392 N m. Any small difference between the products comes from the rule's balance point not being exactly at the pivot and from where the thread loops sit. In the extension, 4.90 N at 10 cm is held level by about 1.2 N at 40 cm (4.90 N x 0.10 m / 0.40 m = 1.225 N), read on the 0 to 2.5 N meter.

What changes

What you change
mass on the right-hand side (or its distance)
What you measure
distance from the pivot at which the rule balances
What you keep the same
  • left-hand load and position
  • the same rule and pivot
  • rule level when reading

Common misconceptions

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

  • The heavier side always goes down regardless of distance.
  • Moving a mass along the rule changes its weight.
  • A lever lets you get more energy out than you put in.

Safety card

Low riskLearners carry it out

Hazards

  • masses sliding off a tipping rule

Controls

  • hold the rule while positioning loads
  • work over the bench

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

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

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-4/fa71c2a852
  3. spark.iop.org/simple-balance-1
  4. spark.iop.org/levers-and-pulleys-multiply-force-not-energy
  5. instructional-resources.physics.uiowa.edu/1j4020-torque-beam

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