Two rod positions, two periods, one honest verdict.
Timed oscillations are the commonest Question 2 in 9702 Paper 3. Here the string of a pendulum wraps round a rod on one side of its swing. Slide the magnifier along the rule to read the cork, the rod and the bob, move the rod by hand, pull the bob aside and time at least five oscillations twice at each rod position. Then test T = k(√L + √(L − d)) against your own criterion and pick out the limitations and improvements that earn marks. Every attempt uses a different pendulum. Allow about 45 minutes.
Past-paper referenceOriginal experiment, wording and numbers, written in the style of Cambridge AS & A Level Physics 9702 Paper 3, Question 2. It is not copied from a past paper.
Practise next: 9702/33 May/June 2018, Question 2 (an interrupted pendulum: the string strikes a rod on one side of the swing) · official past papers
Practise next: 9702/35 May/June 2023, Question 2 (a pendulum hung from cylinders of two diameters, two values of k) · official past papers
This type of experiment has come up 15 times in the past papers we checked. Show the list
Checked: Cambridge 9702 Paper 3, 2017 to 2019 and 2023 to 2025. Only the papers in our collection are listed, so the real number may be higher. Question papers are available from Cambridge.
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How this question is marked
Measurements (a) to (c)
L to the nearest millimetre with a realistic percentage uncertainty (the centre of the bob is judged, so 2 to 5 mm, not 1 mm), d to the nearest millimetre at about half and three-quarters of L, and T from at least two timings of at least five oscillations at each position.
The relationship (d)
Two values of k to two or three significant figures, a percentage difference, a criterion you state yourself, and a conclusion that follows from the comparison.
Limitations (e)(i)
Four different, specific problems with this experiment. “Human error” and “inaccurate equipment” score nothing, and air resistance is not credited.
Improvements (e)(ii)
Four specific changes, each dealing with the limitation it is paired with.
Habits that save marks
Read the top and the bottom of the bob and take the mean: that is why the uncertainty in L is 2 to 5 mm, not 1 mm (examiner reports say most candidates claim too little).
Start the watch as the bob passes the centre, not at the end of a swing, and count at least five complete oscillations twice.
k to one significant figure earns nothing: 0.1 and 0.1 cannot be compared. Give 0.098 and 0.103.
Your conclusion must follow from your own numbers. If the difference is bigger than your criterion, say the data does not support the relationship.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.