Physics 11–12 · Year 11

Standing waves on a string with a vibration generator (progressive against standing waves) (syllabus practical)

Module 3: Waves and Thermodynamics (Wave Behaviour; Sound Waves)

Practical, model not builtLow risk

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The idea

A wave reflected from a fixed end superposes with the incoming wave to give a standing pattern whose allowed frequencies depend on length, tension and mass per unit length.

What you need

Vibration generator driven by a signal generator (0 to 200 Hz)
String of measured mass per unit length (for example 1.0 g/m), pulley, hanger with slotted masses to 1.000 kg
Metre rule, electronic balance reading to 0.01 g, phone slow-motion video

How to do it

Measure 3 m of string on the balance to find mu; set 1.000 m between the generator and the pulley with 0.500 kg hanging.
Sweep the frequency until one loop appears (the fundamental); record f1 and note where the string does not move.
Increase frequency to find two, three and four loops; record each frequency.
Plot f against the number of loops n: the gradient is v/(2L).
Change the hanging mass to 0.250 kg and 1.000 kg and repeat the fundamental; plot f1 against the square root of tension.

What you should see

With L = 1.000 m, T = 0.500 x 9.806 65 = 4.90 N and mu = 1.0 g/m the wave speed is 70.0 m/s and the harmonics are 35.0, 70.0 and 105 Hz. f is proportional to n and to the square root of T; doubling the tension raises the fundamental by a factor of 1.41, so 0.250 kg and 1.000 kg hanging give fundamentals of 24.8 Hz and 49.5 Hz. The learner knows it worked when the measured frequencies agree with these within about 5 per cent, using the vibrating length from the generator to the pulley.

What changes

What you change: driving frequency (and tension in the second series)
What you measure: number of loops and the resonant frequencies
What you keep the same: string length
mass per unit length
same generator amplitude

Common misconceptions

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

A standing wave carries energy along the string; energy is stored in the loops and there is no net transfer.
The points that stay still are where the wave is weakest; they are where two equal waves cancel exactly.

Safety card

Low riskLearners carry it out

Hazards

hanging masses falling
string snapping under load

Controls

keep feet clear beneath the hanger
load no more than the string's rated tension

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 2027PH11-10 PH11/12-3 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities