Explain why a sharp knife cuts through food more easily than a blunt knife of the same weight.

Cambridge International IGCSE Physics (0625) — 1.8 Pressure · Explain · 4 marks · View as Markdown

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

A chef uses two knives of identical mass and blade length. One knife has a very thin, sharp cutting edge whilst the other has a wide, blunt cutting edge.

Model answer (4 marks)

Pressure is force per unit area.
The sharp knife has a smaller contact area at its cutting edge than the blunt knife.
The force (weight/downward force applied) on both knives is the same.
The smaller area of the sharp knife gives a greater pressure on the food, so it cuts more easily.

Examiner tips

  • Use the definition of pressure (F/A) to show why area matters.
  • Show that the force is the same for both knives.
  • Explain that a smaller area gives higher pressure, leading to easier cutting.
  • Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Confusing mass with force – forgetting that the force is the weight of the knife.
  • Not recognising that the contact area is the key difference between the knives.
  • Using vague phrases like "more efficient" instead of the precise pressure explanation.

Mark scheme (4 marks)

  1. Pressure is defined as force per unit area (or equivalent statement linking pressure to force and area)
  2. The sharp knife has a smaller (contact) area at its cutting edge than the blunt knife
  3. The force (weight/downward force applied) acting on both knives is the same
  4. The smaller area of the sharp knife produces a greater pressure on the food, so it cuts more easily

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