Physics 11–12 · Year 12
Centripetal force against mass, speed and radius with a whirling rubber stopper (syllabus practical)
Module 5: Advanced Mechanics (Circular Motion)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
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The idea
An object moving in a circle needs a net force towards the centre equal to m v squared over r, which a hanging weight supplies through a string.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- stopper striking someone
- string breaking under load
- masses falling
Controls
- 3 m clear radius, safety glasses for everyone nearby
- inspect the string, load no more than 0.400 kg
- whirl over the head with the tube vertical
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/).
What you need
- Rubber stopper (about 50 g) tied to 1.5 m of strong string threaded through a smooth plastic tube, with a hanger and slotted masses (0.100 to 0.400 kg) on the lower end
- Paperclip marker on the string below the tube, stopwatch, metre rule, electronic balance, safety glasses
How to do it
- Weigh the stopper. Hang 0.200 kg on the lower end and set the radius to 0.800 m by positioning the paperclip just below the tube.
- Whirl the stopper in a horizontal circle so the paperclip stays steady; time 20 revolutions, three times.
- Compute v = 2 pi r / T and compare m v^2 / r with the hanging weight M g.
- Series 1: change the hanging mass (0.100 to 0.400 kg) at fixed radius; plot the hanging weight against v^2.
- Series 2: change the radius (0.4 to 1.0 m) at fixed hanging mass; plot v^2 against r.
What you should see
With a 0.050 kg stopper, 0.200 kg hanging and r = 0.800 m the stopper circles at 5.60 m/s with a period of 0.897 s, and m v^2 / r = 1.96 N equals the hanging weight. Hanging weight against v^2 is a straight line of gradient m/r; v^2 against r is a straight line of gradient M g / m. The learner knows it worked when m v^2 / r and M g agree within about 10 per cent. The string droops below horizontal, but the tension along the full string length L still obeys M g = m (2 pi / T)^2 L, so the measured string length is the right radius to use.
What changes
- What you change
- hanging mass (centripetal force), then radius
- What you measure
- speed of the stopper
- What you keep the same
- stopper mass
- radius (series 1) or hanging mass (series 2)
- horizontal circle
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A centrifugal force pushes the stopper outward; the only horizontal force on the stopper is the inward tension.
- If the string breaks the stopper flies outward along the radius; it moves off along the tangent.
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 2027PH12-12PH11/12-5
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-01PY-12WS-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.