Physics 11–12 · Year 11
Coefficient of kinetic friction with a spring balance
Module 2: Dynamics (Forces, Acceleration and Energy)
The idea
The kinetic friction force between two surfaces grows in proportion to the normal force, and the constant of proportionality is a property of the pair of surfaces.
What you need
- Steel retort stand base (about 1.3 kg) or a wooden slider, and three house bricks of about 2.85 kg each, added one at a time (the guide's loads are 1.3, 4.2, 7.0 and 9.8 kg)
- Spring balance reading to 50 N, or a force sensor with data logger
- String, electronic balance or bathroom scales for the loaded masses
- Laminate benchtop as the test surface (the NSW Department of Education Module 2 guide activity)
How to do it
- Weigh the slider; tie the string to it and hook on the balance, keeping the string horizontal.
- Pull at a slow constant speed and read the balance while sliding; take three readings and average.
- Add one brick at a time (record each total mass) and repeat.
- Compute the normal force N = m g for each load; plot friction force against N.
- The gradient is the coefficient of kinetic friction; discuss whether the line passes through the origin and whether the model fits.
What you should see
Friction force rises in proportion to normal force. With the retort stand and three bricks (9.8 kg total) the normal force is 96.1 N, and the Module 2 guide's spring-scale reading at that load is 21 plus or minus 2 N. Plotting friction against normal force gives the coefficient straight from the gradient; the guide plots friction against mass, reads a gradient of 2.207 N/kg, which is the coefficient times g, and reports a coefficient of kinetic friction of 2.207 / 9.8 = 0.23 for a steel retort stand on a laminate benchtop. The same guide also shows where the model fails: a foam-covered board's friction rose by only 50 per cent when its load almost doubled from 3.7 kg to 6.5 kg. The learner knows it worked when the points lie on a straight line with an intercept near zero.
What changes
- What you change
- normal force (load)
- What you measure
- kinetic friction force
- What you keep the same
- same pair of surfaces
- constant slow speed
- horizontal string
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Friction depends on the area of contact; for these surfaces it depends on the normal force, not the area.
- A heavier object needs a larger pull at constant speed because of its inertia; at constant speed there is no acceleration, and the larger pull balances the larger friction that its larger normal force produces.
Safety card
Hazards
- bricks falling on feet
- string snapping
Controls
- load bricks squarely
- closed shoes
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 2027PH11-9PH11/12-4
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-01PY-11WS-05
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/Physics-module-2-guide.docx
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes