A car is travelling along a straight, level road at a constant speed. The driver then presses the accelerator pedal and the car speeds up. Explain, using Newton's laws, why the car travels at a constant speed before the accelerator is pressed, and why it then accelerates when the driver presses the accelerator pedal.

WJEC A-Level Physics (Wales) — 1.2 Forces and Newton's laws · 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 speed the driving force from the engine is exactly balanced by the resistive forces such as friction and air resistance, so the net force is zero. By Newton’s first law an object that has no resultant force continues at a constant velocity, which is why the car keeps moving at a constant speed.

When the accelerator is pressed the engine produces a larger driving force. Now the driving force exceeds the resistive forces, giving a non‑zero resultant force in the forward direction. According to Newton’s second law this resultant force causes the car to accelerate in that direction.

Examiner tips

  • Show the balance of forces at constant speed and the imbalance after the accelerator is pressed.
  • Mention Newton’s first law for the constant‑speed case and Newton’s second law for the acceleration.
  • Use the terms ‘driving force’, ‘resistive forces’, ‘net force’ and ‘resultant force’.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Failing to state that the net force is zero at constant speed.
  • Confusing the direction of the resultant force with the direction of acceleration.
  • Using vague language such as ‘the car moves’ instead of ‘the car has a net force of zero’.

Mark scheme (5 marks)

  1. At constant speed, the driving force equals the resistive forces (e.g. friction/air resistance)
  2. When forces are balanced the resultant force is zero
  3. Newton's first law: an object continues at constant velocity (or remains stationary) unless acted on by a resultant force
  4. When the accelerator is pressed, the driving force increases so the driving force is greater than the resistive forces, producing a resultant force
  5. Newton's second law: a resultant force causes acceleration (in the direction of the resultant force)

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