A car is travelling along a straight, level road at a constant speed. The driver then presses the accelerator pedal harder, causing the car to speed up. Explain why the car first travels at constant speed and then accelerates when the driver presses the accelerator harder.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car travels at constant speed along a level road. The driver then presses the accelerator pedal harder and the car speeds up.
Model answer (4 marks)
At constant speed the driving force from the engine equals the resistive forces (friction and air resistance). A resultant force of zero means no change in motion, so the car keeps its speed.
When the driver presses the accelerator harder the engine produces a larger driving force. Now the driving force exceeds the resistive forces, giving a net force in the direction of motion. This non‑zero resultant force causes the car to accelerate and its speed increases.
When the driver presses the accelerator harder the engine produces a larger driving force. Now the driving force exceeds the resistive forces, giving a net force in the direction of motion. This non‑zero resultant force causes the car to accelerate and its speed increases.
Examiner tips
- Use the term ‘driving force’ and ‘resistive forces’ explicitly.
- Show the logic: equal forces → zero net force → no acceleration; greater driving force → net force → acceleration.
- Mention the direction of the net force (forward).
Common mistakes
- Confusing ‘speed’ with ‘velocity’ or not recognising the role of resistive forces.
- Failing to state that a zero net force gives no acceleration or that a non‑zero net force causes acceleration.
Mark scheme (4 marks)
- At constant speed the driving force equals the resistive forces (friction/air resistance)
- A resultant (net) force of zero means there is no change in motion / no acceleration
- Pressing the accelerator increases the driving force so the driving force is now greater than the resistive forces
- A resultant force in the direction of motion causes the car to accelerate (speed up)
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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