Physics 11–12 · Year 12
Torque and rotational equilibrium with a metre rule on a pivot
Module 5: Advanced Mechanics (Circular Motion)
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The idea
A force turns an object about a pivot with a torque equal to force times perpendicular distance, and a body stays balanced when the torques cancel.
What you need
- Metre rule with a knife-edge pivot or a hole at the 50 cm mark on a stand
- Slotted 50 g and 100 g masses with hangers, string loops, spring balance, protractor
How to do it
- Balance the rule on its pivot alone; hang 0.200 kg at 30.0 cm from the pivot and find where a 0.400 kg load balances it.
- Record five different balancing pairs and compute clockwise and anticlockwise torques.
- Hang the 0.200 kg mass and pull upward with the spring balance at 20.0 cm from the pivot at 90 degrees, then at 60 and 45 degrees to the rule; record the force needed to balance each time.
- Plot the balancing force against 1 over sin(angle).
What you should see
0.200 kg at 0.300 m gives a torque of 0.588 N m, balanced by 0.400 kg at 0.150 m; the learner knows it worked when the clockwise and anticlockwise torque sums agree within about 2 per cent. Pulling at 0.200 m from the pivot needs 2.94 N at 90 degrees, 3.40 N at 60 degrees and 4.16 N at 45 degrees, 1.41 times the perpendicular value.
What changes
- What you change
- distance of the load from the pivot (or angle of the applied force)
- What you measure
- balancing force
- What you keep the same
- load mass
- pivot position
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A larger force always gives a larger turning effect; a small force far from the pivot can beat a large force near it.
- Torque depends on the length of the object; it depends on the perpendicular distance from the pivot to the force's line of action.
Safety card
Hazards
- masses falling
Controls
- hang masses close to the bench
Note
Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-12PH11/12-4PH11/12-6
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12WS-04
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.