A car is travelling along a motorway at a constant speed. Explain why the car must have its engine running to maintain this constant speed.

AQA GCSE Physics (8463) — 4.5.6 Forces and motion · Explain · 4 marks · View as Markdown

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

A car travels at a constant speed of 70 mph along a straight, level motorway.

Model answer (4 marks)

A car moving at a constant speed must have its engine running because there is a resistive force (friction and air resistance) that opposes its motion. The engine supplies a driving force in the forward direction that balances this resistive force. When the driving force equals the resistive force, the resultant force on the car is zero. According to Newton’s First Law, a zero resultant force means the car continues to move at a constant speed.

Examiner tips

  • Use the word ‘resistive force’ and specify friction/air resistance.
  • State that the engine provides a forward driving force.
  • Show that driving force equals resistive force → resultant force zero.
  • Mention Newton’s First Law to link zero resultant force to constant speed.

Common mistakes

  • Failing to mention the resistive force or the engine’s driving force.
  • Confusing ‘constant speed’ with ‘constant velocity’ and not linking to Newton’s First Law.
  • Using vague terms like ‘push’ instead of ‘force’ or ‘drive’.

Mark scheme (4 marks)

  1. There is a resistive / friction / air resistance force acting on the car (opposing motion)
  2. The engine provides a driving force (forward / in the direction of motion)
  3. The resultant force on the car is zero (driving force equals resistive force)
  4. By Newton's First Law, a zero resultant force means the car continues at constant speed / velocity

Key terms in this question

constant speed

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