A car is travelling along a straight, level road. The driver presses the accelerator pedal so that the driving force from the engine becomes greater than the total resistive forces acting on the car. Explain what happens to the motion of the car, and describe what must happen to the forces for the car to then travel at a new, higher constant speed.

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)

When the driving force exceeds the resistive forces, a net forward force acts on the car, so it accelerates and its speed increases.

As the car speeds up, the resistive forces (air resistance, rolling friction, etc.) also rise.

To reach a new constant speed, the driving force must be adjusted so that it equals the now‑larger resistive forces, giving a net force of zero and stopping further acceleration.

Examiner tips

  • Use the term 'net force' or 'resultant force' to show understanding of Newton’s first law.
  • Explain the feedback loop: speed up → resistive forces up → forces must balance for constant speed.
  • Mention that the car’s acceleration is positive when driving force > resistive forces.
  • Show the final state: driving force = resistive forces → zero net force → constant speed.

Common mistakes

  • Confusing acceleration with constant speed; forgetting that resistive forces increase with speed.
  • Using vague terms like 'more force' instead of specifying 'net force' or 'resultant force'.
  • Failing to state that the driving force must equal the increased resistive forces for constant speed.

Mark scheme (4 marks)

  1. When the driving force is greater than the resistive forces, there is a resultant force acting on the car
  2. A resultant force causes the car to accelerate (speed up)
  3. As speed increases, resistive forces (e.g. air resistance/friction) also increase
  4. For constant speed, the driving force must equal the (increased) resistive forces so the resultant force is zero

Key terms in this question

driving force · resistive forces · constant speed

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