Hang it, read it twice. Extension is a difference.
Stretching cords, wires and strips is a regular Question 1 family in Paper 5. Here the same load hangs on rubber cords of different diameter d, and you measure the extension e to test e = p dq and find the Young modulus. 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: 9702/52 February/March 2018, Question 1 (extension of elastic cords of different diameters) · official past papers
Practise next: 9702/52 February/March 2016, Question 1 (bungee model: elastic cords with different spring constants) · official past papers
Practise next: 9702/51 May/June 2019, Question 1 (bending of a loaded wooden strip) · official past papers
Examiner reports used: 9702 March 2018 and June 2019 (there is no report for March 2016).
This set-up builder needs JavaScript.
What a full-mark diagram shows
The support and the load
The cord clamped to a stand at the top, with labelled slotted masses hanging from its lower end. Draw the bench, not apparatus in mid-air.
Measuring the extension
A metre rule clamped vertically beside the cord, checked with a set square. Read the pointer with no load and with the load; e is the difference.
Measuring d and F
A micrometer for d, repeated along the cord and averaged. A balance for the mass M of the load, then F = Mg.
The analysis
lg e against lg d: valid if the graph is a straight line, q is the gradient, and E = 4MgL / (πp) with p = 10y-intercept.
Habits the examiner reports keep asking for
Write “kept constant”, not “control”: keep M and the unstretched length L constant.
“Measured with a ruler” is not enough. Give both readings and subtract to find e.
A balance gives mass, not force. Multiply by g to get F.
Additional detail must be a method: measure d at several points and average, wait for the load to stop bouncing, check the pointer returns when the load is removed.
Safety needs a reason linked to this experiment: a sand tray in case the load falls, goggles in case the cord snaps back.