A student pushes a heavy box along a flat floor at a constant velocity. Explain why the box moves at constant velocity even though the student is applying a force to it.

Eduqas GCSE Physics — 3.1 Forces and their interactions · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A student pushes the box, but friction (or air resistance) acts in the opposite direction. The pushing force and the friction force are equal in size, so the forces are balanced. A zero resultant force means there is no change in motion, and the box therefore continues at a constant velocity.

Examiner tips

  • Use the word ‘balance’ to show forces are equal and opposite.
  • Mention Newton’s First Law to justify constant velocity.
  • Keep the answer concise – 4 short sentences are enough.
  • Use UK spelling (e.g., ‘friction’).

Common mistakes

  • Forgetting to mention friction or air resistance.
  • Saying the forces are ‘equal’ but not ‘opposite’ direction.
  • Confusing ‘constant velocity’ with ‘zero velocity’ or ‘no motion’.

Mark scheme (4 marks)

  1. Friction (or air resistance) acts on the box in the opposite direction to the push
  2. The pushing force and the friction force are equal in size
  3. The forces are balanced (resultant/net force is zero)
  4. A zero resultant force means there is no change in motion / the box continues at constant velocity (Newton's First Law)

Key terms in this question

constant velocity

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