A wooden block is placed on a rough horizontal surface. A student pushes the block with a horizontal force, but the block does not move. Explain why the block remains stationary, using the concept of resultant force.

Cambridge International IGCSE Physics (0625) — 1.5 Forces · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. The applied horizontal force is opposed by a frictional force from the rough surface.
2. The frictional force is equal in magnitude to the applied force, so the two forces cancel.
3. The vector sum of the forces – the resultant force – is therefore zero.
4. With a zero resultant force the block experiences no net force and hence no acceleration, so it remains stationary.

Examiner tips

  • Use the word ‘friction’ and state its direction opposite to the applied force. Mention that the magnitudes are equal. Explain that a zero resultant force gives zero acceleration. Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing static and kinetic friction; not recognising the forces are equal. Failing to state that the resultant force is zero. Using vague terms like ‘no force’ instead of ‘zero resultant force’.

Mark scheme (4 marks)

  1. Friction acts on the block in the opposite direction to the applied force
  2. Friction is equal in magnitude to the applied force
  3. The resultant force on the block is zero
  4. A zero resultant force means the block remains stationary / does not accelerate

Key terms in this question

resultant force

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