A car is travelling along a straight, level road at a constant speed. Explain why the car continues at constant speed even though the engine is providing a driving force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car engine produces a driving force to keep the car moving at a steady speed along a flat road.
Model answer (4 marks)
At constant speed the forces on the car are balanced – the resultant force is zero.
The engine supplies a driving force to the rear wheels.
Opposing this is a resistive force (friction, air resistance, rolling resistance) acting in the opposite direction.
The driving force equals the resistive force in magnitude, so the net force is zero.
By Newton’s first law, with no resultant force the car continues at constant velocity.
The engine supplies a driving force to the rear wheels.
Opposing this is a resistive force (friction, air resistance, rolling resistance) acting in the opposite direction.
The driving force equals the resistive force in magnitude, so the net force is zero.
By Newton’s first law, with no resultant force the car continues at constant velocity.
Examiner tips
- Use the word ‘balanced’ to show forces are equal and opposite.
- Mention the specific resistive forces (friction, air resistance).
- State Newton’s first law explicitly.
- Keep the answer concise – 4 short sentences or points.
Common mistakes
- Failing to mention the resistive force or calling it ‘friction’ only.
- Not stating that the driving force equals the resistive force.
- Using ‘speed’ instead of ‘velocity’ and ignoring Newton’s first law.
Mark scheme (4 marks)
- At constant speed, the forces on the car are balanced (resultant force is zero)
- There is a resistive force / friction / air resistance acting on the car in the opposite direction to motion
- The driving force equals the resistive force / friction / air resistance in size
- According to Newton's first law, an object continues at constant velocity when the resultant force is zero / no resultant force means no change in motion
Key terms in this question
driving force · constant speed
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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