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 why the car travels at constant speed initially, and then describe what happens to the forces acting on the car when it accelerates.

Eduqas GCSE Physics — 4.2 Forces, accelerations and Newton's laws of motion · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

At constant speed the driving force from the engine is balanced by the resistive forces (rolling friction and air resistance), so the net force is zero.

Because the resultant force is zero, the car does not accelerate (Newton’s First Law).

When the accelerator is pressed the engine produces a larger driving force, which becomes greater than the resistive forces.

Now there is a net force in the direction of motion, giving the car a positive acceleration.

Examiner tips

  • Use the word ‘balanced’ or ‘equal’ for the forces at constant speed. Show the chain: driving force > resistive forces → resultant force → acceleration. Mention Newton’s First Law when explaining no acceleration.
  • Include the direction of the resultant force (forward) to show it causes acceleration.

Common mistakes

  • Saying the car accelerates before the accelerator is pressed. Forgetting to state that the resistive forces are rolling friction and air resistance. Using vague terms like ‘more force’ without specifying which forces change.

Mark scheme (4 marks)

  1. At constant speed the driving force and resistive forces (friction/air resistance) are balanced / equal
  2. Because the resultant force is zero, the car does not accelerate (Newton's First Law)
  3. When the accelerator is pressed, the driving force increases / becomes greater than the resistive forces
  4. There is now a resultant force in the direction of motion, causing the car to accelerate

Key terms in this question

constant speed

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