A car accelerates from rest along a straight, level road. The car's engine provides a constant thrust force. Explain why the car first accelerates rapidly, then accelerates more slowly, and finally reaches a constant speed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car's engine provides a constant thrust force. At first, the car speeds up quickly. Over time, the acceleration decreases. Eventually, the car travels at a constant speed.
Model answer (5 marks)
At the start the car is almost stationary, so the frictional force of the tyres and the aerodynamic drag are very small. The engine thrust therefore gives a large resultant force, and by F=ma the acceleration is large.
As the car speeds up, the drag force (which increases roughly with the square of speed) grows. The resultant force (thrust minus drag and friction) becomes smaller, so the acceleration decreases.
When the thrust equals the sum of the drag and friction forces the resultant force is zero. The car then has no further acceleration and moves at a constant terminal speed.
As the car speeds up, the drag force (which increases roughly with the square of speed) grows. The resultant force (thrust minus drag and friction) becomes smaller, so the acceleration decreases.
When the thrust equals the sum of the drag and friction forces the resultant force is zero. The car then has no further acceleration and moves at a constant terminal speed.
Examiner tips
- Show the sequence of forces: thrust, friction, drag
- Explain how the drag increases with speed
- Use F=ma to link force to acceleration
- Mention terminal speed when forces balance
Common mistakes
- Ignoring the increase of drag with speed
- Confusing friction with drag
- Failing to state that acceleration becomes zero at terminal speed
Mark scheme (5 marks)
- At the start, friction and drag (air resistance) are small, so the resultant force is large
- A large resultant force causes a large acceleration (linked to F = ma)
- As speed increases, air resistance / drag increases
- Increasing drag reduces the resultant force, so acceleration decreases
- Terminal velocity is reached when thrust equals drag and friction, giving a zero resultant force and no acceleration, so constant speed
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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