The pendulum is one of the most common Paper 6 experiments. A small bob swings on a thread, you change its length l and you find the period T, the time for one complete oscillation. 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 points examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge IGCSE Physics 0625 Paper 6 (Alternative to Practical). 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: 0625/62 October/November 2023, (period of a pendulum, testing whether T² is proportional to d) · official past papers
Practise next: 0625/61 October/November 2024, (period of a pendulum, techniques and choosing lengths) · official past papers
Practise next: 0625/62 May/June 2024, (timing 20 oscillations of a mass on a spring) · official past papers
Examiner reports used: 0625 November 2023, June 2024, November 2024, March 2025 and June 2025.
This set-up builder needs JavaScript.
What a full-mark diagram shows
A fixed point at the top
The thread is clamped between split corks, so it swings from one point and the length does not change.
Length to the centre of the bob
A metre rule held vertically close to the thread, with a set square level with the centre of the bob as a horizontal aid.
A fiducial marker
A mark at the centre of the swing, where the bob moves fastest. Start and stop timing as the bob passes it, counting from zero.
Many oscillations, small angle
A stopwatch times 20 oscillations of a small swing, then T = t ÷ 20.
Habits the examiner reports keep asking for
Record the stopwatch exactly, every digit as a decimal in seconds: 28.46, not 28 or 28:46.
Put units in the headings: l / cm, t / s, T / s and T² / s². Many candidates write s for T².
Timing 20 oscillations does not shorten your reaction time. It spreads the same reaction-time error over a longer time, so it matters less.
Say how you use a set square, not just “use a set square”: its edge against the rule, level with the centre of the bob.
Directly proportional means a straight line through the origin, or a constant ratio T²/l. “Both increase” is not enough.