A car accelerates from rest along a straight, level road. The engine provides a constant thrust force. Explain why the car's acceleration decreases over time, even though the thrust force remains constant, and describe what happens to the car's motion when the resultant force becomes zero.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car pulls away from a set of traffic lights and accelerates along a straight, level road. The engine provides a constant thrust force throughout.
Model answer (4 marks)
As the car speeds up, the drag force (air resistance) increases.
The resultant force on the car therefore decreases because the thrust remains constant while the drag increases, so the forces become more balanced.
Since the resultant force equals mass times acceleration (F = ma), a smaller resultant force means a smaller acceleration.
When the resultant force becomes zero (thrust equals drag plus any other resistive forces), the car reaches its terminal velocity and travels at a constant speed.
The resultant force on the car therefore decreases because the thrust remains constant while the drag increases, so the forces become more balanced.
Since the resultant force equals mass times acceleration (F = ma), a smaller resultant force means a smaller acceleration.
When the resultant force becomes zero (thrust equals drag plus any other resistive forces), the car reaches its terminal velocity and travels at a constant speed.
Examiner tips
- Use the order: drag increases → resultant force ↓ → acceleration ↓. Mention the equation F = ma to link force and acceleration. State that zero resultant force gives terminal velocity and constant speed.
Common mistakes
- Confusing drag with friction. Forgetting to mention that the car reaches terminal velocity when forces balance. Using vague terms like "slows down" instead of "acceleration decreases".
Mark scheme (4 marks)
- As the car speeds up, drag (air resistance) increases
- The resultant force decreases because thrust remains constant but drag increases, so the forces become more balanced
- Because resultant force = mass × acceleration (F = ma), a smaller resultant force means smaller acceleration
- When the resultant force is zero (thrust equals drag plus friction / forces are balanced), the car reaches terminal velocity and travels at a constant speed
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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