A car is travelling along a straight, level road at a constant velocity. The driver then presses the accelerator pedal and the car accelerates. Explain, using Newton's laws of motion, why the car travels at constant velocity before the driver presses the accelerator, and why the car accelerates after the driver presses the accelerator.
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 such as friction and air resistance, so the net force is zero.
Because the net force is zero, Newton’s First Law tells us that the car’s velocity does not change.
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 direction of motion.
According to Newton’s Second Law a non‑zero resultant force causes the car to accelerate in that direction.
Because the net force is zero, Newton’s First Law tells us that the car’s velocity does not change.
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 direction of motion.
According to Newton’s Second Law a non‑zero resultant force causes the car to accelerate in that direction.
Examiner tips
- Use the exact terms ‘driving force’, ‘resistive forces’, ‘net force’, ‘Newton’s First/Second Law’.
- Show the logical sequence: equal forces → zero net → no change; increased force → non‑zero net → acceleration.
- Keep each point short and directly linked to the mark scheme points.
Common mistakes
- Confusing ‘constant velocity’ with ‘zero velocity’; students say the car is at rest. Forgetting to mention the resistive forces or the net force being zero. Mixing up Newton’s First and Second Laws or not stating that a resultant force causes acceleration.
Mark scheme (5 marks)
- At constant velocity, the driving force equals the resistive forces (e.g. friction/air resistance)
- A resultant force of zero means there is no change in motion (Newton's First Law)
- When the accelerator is pressed, the driving force increases
- The driving force is now greater than the resistive forces, so there is a resultant force in the direction of motion
- A resultant force causes acceleration in the direction of that force (Newton's Second Law)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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