A student pushes a heavy box along a warehouse floor at a constant velocity. Explain why the box moves at a constant velocity even though the student is applying a force, and describe what would happen to the motion of the box if the student suddenly doubled their pushing force.

OCR A-Level Physics B: Advancing Physics (H557) — 2.1 Physical quantities and units · Explain · 5 marks · View as Markdown

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

A student applies a horizontal pushing force to a heavy box resting on a concrete warehouse floor. The box moves at a constant velocity across the floor.

Model answer (5 marks)

At constant velocity the resultant force on the box is zero – the horizontal pushing force is exactly balanced by the kinetic friction force acting in the opposite direction. This balance of forces means the net force is zero, so by Newton’s First Law the box continues to move at a constant velocity.

If the student suddenly doubled the pushing force, the pushing force would exceed the friction force. A non‑zero resultant force would then act in the direction of the push, giving the box a net acceleration. Consequently the box would start to speed up in the direction of the push, in accordance with Newton’s Second Law.

Examiner tips

  • Show the force balance first, then link to Newton’s First Law for constant velocity. Mention kinetic friction as the opposing force. For the doubled force, state the new net force and that it causes acceleration. Use the correct terminology: resultant force, net force, kinetic friction, acceleration.

Common mistakes

  • Failing to mention the opposing friction force or the resultant force being zero. Treating constant velocity as evidence of zero force without reference to Newton’s First Law. Assuming the box would stop or move faster without explaining the role of friction and net force.

Mark scheme (5 marks)

  1. At constant velocity, the resultant force on the box is zero / the forces are balanced
  2. The student's pushing force is equal to the friction force acting in the opposite direction
  3. This is consistent with Newton's First Law: an object with zero resultant force continues at constant velocity
  4. When the pushing force is doubled, a resultant force now acts on the box in the direction of motion (pushing force is greater than friction)
  5. The box will accelerate / speed up (in the direction of the push), as stated by Newton's Second Law

Key terms in this question

constant velocity

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