Physics 11–12 · Year 11
Resonance in mechanical systems: driven pendulums and resonant rings (syllabus practical)
Module 3: Waves and Thermodynamics (Wave Behaviour)
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The idea
A system driven near its natural frequency absorbs energy each cycle and builds a large amplitude, while driving well above or below that frequency produces little response.
What you need
- Barton's pendulums: a taut string with five paper-cone pendulums of lengths 0.20 to 0.60 m and one heavy driver pendulum of 0.40 m
- Resonant rings: loops of paper or card of different diameters taped to a card base (five rings in the NSW Department of Education Module 3 guide, Activity 3), shaken by hand or on a shaker
- Stopwatch, metre rule, phone video
How to do it
- Time ten swings of each light pendulum to get its natural frequency.
- Set the heavy driver swinging and wait a minute; observe which light pendulum builds the largest amplitude.
- Change the driver length and repeat; the pendulum of matching length responds most.
- Resonant rings: shake the base slowly, then faster; note that each ring vibrates strongly at its own frequency.
- Plot response amplitude against driving frequency for the ring set to sketch a resonance curve.
What you should see
The light pendulum whose length equals the driver's (0.40 m, natural frequency 0.79 Hz) swings with the largest amplitude and lags the driver by a quarter cycle; longer and shorter pendulums swing little. The rings show the same selectivity: the largest ring responds at the lowest shake rate. The response curve peaks at the natural frequency and narrows when damping is reduced.
What changes
- What you change
- driving frequency
- What you measure
- steady-state amplitude of the driven pendulum
- What you keep the same
- driver amplitude
- pendulum masses
- string tension
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A stronger push always gives a bigger swing; timing near the natural frequency matters more than push size.
- Resonance is a property of the driver; it is a property of the driven system.
Safety card
Hazards
- heavy driver pendulum swinging into a face
Controls
- stand behind the frame
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.
Sources
The pages the author read to write this activity.