A nylon climbing rope and a cast iron rod are both subjected to increasing tensile forces until they fail. The nylon rope stretches considerably before breaking, whilst the cast iron rod snaps with very little extension. Explain the difference in behaviour of these two materials in terms of their mechanical properties.

AQA A-Level Physics (7408) — 3.4.2 Materials · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The nylon rope is ductile – it undergoes a large plastic deformation before breaking and does not return to its original length. The cast iron rod is brittle – it fractures with little or no plastic deformation, snapping suddenly with very little extension. The cast iron also has a higher stiffness, so a greater force is required to produce the same extension as the nylon rope.

Examiner tips

  • Use the terms ‘ductile’ and ‘brittle’ as they are key words in the mark scheme.
  • Explain that ductile materials deform plastically before failure, while brittle materials fracture with little deformation.
  • Mention the higher stiffness of cast iron to justify the larger force needed for the same extension.
  • Keep the answer concise – 5 marks can be earned with 3–4 short points.

Common mistakes

  • Using ‘elastic’ instead of ‘ductile’ for nylon, or ‘elastic’ for cast iron.
  • Failing to mention the large plastic deformation of nylon or the lack of it in cast iron.
  • Omitting the higher stiffness of cast iron or confusing stiffness with strength.

Mark scheme (5 marks)

  1. The nylon rope is described as ductile (or malleable / not brittle)
  2. The nylon rope undergoes a large (plastic) deformation before breaking / it does not return to its original length
  3. The cast iron rod is described as brittle
  4. A brittle material breaks / fractures with little or no plastic deformation (snaps suddenly without stretching significantly)
  5. The cast iron has a higher stiffness (than nylon) / a greater force is needed to produce the same extension

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