A car is travelling along a straight, level road at a constant velocity of 20 m/s. The driver sees an obstacle ahead and applies the brakes. The car decelerates uniformly and comes to rest. Explain, in terms of forces, why the car travels at constant velocity before the brakes are applied, and then explain why the car decelerates once the brakes are applied.

AQA A-Level Physics (7408) — 3.4.1 Mechanics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

At constant velocity the driving force from the engine equals the resistive forces (rolling friction and air resistance). The resultant force is therefore zero, so by Newton’s first law the car continues with constant speed.

When the brakes are applied a braking force (friction between brake pads and wheels) acts opposite to the direction of motion. This creates a non‑zero resultant force in the opposite direction, giving a negative acceleration (deceleration) and the car slows until it stops.

Examiner tips

  • State the balance of forces before braking; mention engine force equals resistive forces.
  • Explain that zero resultant force gives no acceleration (Newton’s 1st law).
  • Describe the braking force as opposite to motion and its effect on acceleration.
  • Use correct terminology: braking force, resultant force, deceleration.

Common mistakes

  • Failing to mention the engine force or resistive forces before braking.
  • Confusing acceleration with speed; not recognising zero resultant force gives no acceleration.
  • Using vague terms like ‘force’ without specifying direction or type of force.

Mark scheme (5 marks)

  1. At constant velocity, the driving force equals the resistive forces / friction and air resistance
  2. Because the resultant force is zero, there is no acceleration (Newton's first law)
  3. When brakes are applied, a braking force / friction acts on the car (in the opposite direction to motion)
  4. This produces a resultant force in the opposite direction to motion
  5. A resultant force causes acceleration (deceleration), so the car slows down and stops

Key terms in this question

constant velocity

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