A shopping trolley is pushed along a flat supermarket aisle. The shopper pushes the trolley with a constant force. Explain why the trolley moves at a constant velocity rather than continuing to accelerate, and explain what would happen to the trolley's motion if the shopper suddenly stops pushing.

OCR GCSE Physics A: Gateway Science (J249) — P2.2 Newton's laws · Explain · 4 marks · View as Markdown

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

A shopper pushes a loaded shopping trolley along a flat supermarket aisle. Despite a constant pushing force being applied, the trolley moves at a constant velocity. When the shopper lets go, the trolley gradually slows down and stops.

Model answer (4 marks)

The shopper applies a constant pushing force, but friction (and air resistance) acts in the opposite direction. The two forces are equal in magnitude and opposite in direction, so the resultant force is zero. By Newton’s First Law, an object with zero resultant force moves at a constant velocity, so the trolley keeps moving at a constant speed.

If the shopper suddenly stops pushing, the frictional force is no longer balanced. A net force now acts backwards, giving the trolley a negative acceleration. The trolley therefore decelerates and eventually comes to rest.

Examiner tips

  • Show the two forces and state they are equal and opposite to get the zero resultant force point.
  • Use Newton’s First Law to link zero resultant force to constant velocity.
  • Explain the change in forces when the push stops and the resulting deceleration.
  • Use clear, exam‑style language and include the key terms: friction, resultant force, Newton’s First Law, decelerate.

Common mistakes

  • Forgetting to mention friction or air resistance as the opposing force.
  • Failing to state that the resultant force is zero before the push stops.
  • Using vague language such as “the trolley slows down” without explaining the force change.

Mark scheme (4 marks)

  1. Friction (and/or air resistance) acts on the trolley in the opposite direction to its motion
  2. The pushing force is balanced by friction, giving a resultant force of zero
  3. By Newton's First Law, an object with zero resultant force continues at constant velocity
  4. When the shopper stops pushing, friction is no longer balanced so there is a resultant force acting backwards, causing the trolley to decelerate and stop

Key terms in this question

constant velocity

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