Shake the spring, find the peak. Read f on a c.r.o.
Oscillation plans driven by a vibration generator turn up regularly in Paper 5. Here a signal generator drives a vibrator that shakes the top of a spring, and for each load mass m you find the resonant frequency f, where the load swings with its largest amplitude. Watch the set-up built step by step, with the variables and the graph beside it, then build it yourself and check it against the mark points.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Physics 9702 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9702/51 May/June 2015, Question 1 (simple harmonic motion of a mass on an electric vibrator) · official past papers
Practise next: 9702/51 May/June 2014, Question 1 (period of a ball rolling on a curved track) · official past papers
Practise next: 9702/51 May/June 2019, Question 1 (bending of a loaded wooden strip, with a vertical rule) · official past papers
Examiner reports used: 9702 June 2014, June 2015 and June 2019.
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What a full-mark diagram shows
The drive circuit
A signal generator joined to the vibration generator by two leads. Never a d.c. supply.
Measuring f
A c.r.o. in parallel with the vibrator: one cycle × time-base gives T, then f = 1/T. Not a stop-watch.
Judging the amplitude
A metre rule clamped vertically beside the load, checked with a set square, with a pointer and a fixed marker.
Measuring m
Name a balance for the load mass. The word “scale” on its own is too vague.
Habits the examiner reports keep asking for
Use the word “constant”, not “controlled”: keep the same spring, and keep the peak p.d. across the vibrator constant.
Explain how you find resonance: change f slowly from below and from above until the amplitude against the rule is largest.
Say exactly what you repeat: find f again for each m and average, not just “repeat the experiment”.
Safety must fit this experiment: a sand tray or cushion under the load in case masses fly off at resonance.
Name the axes and make k and c the subjects. 1/f² against m is valid if it is a straight line with a positive y-intercept.