A passenger train starts from rest at a station and speeds up along a straight track. After some time, the train travels at a constant velocity. Explain why the train's velocity stops increasing even though the engine continues to provide a driving force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A train leaves a station and accelerates along a straight, level track. The engine provides a constant driving force throughout the journey. After a period of acceleration, the train reaches a constant velocity and no longer speeds up.
Model answer (4 marks)
As the train speeds up, resistive forces such as friction, air resistance and drag increase.
When the train reaches a constant velocity, the driving force from the engine is exactly balanced by these resistive forces, so the resultant force is zero.
With no resultant force, the train does not accelerate (Newton’s First Law) and therefore continues to travel at a constant, terminal velocity.
When the train reaches a constant velocity, the driving force from the engine is exactly balanced by these resistive forces, so the resultant force is zero.
With no resultant force, the train does not accelerate (Newton’s First Law) and therefore continues to travel at a constant, terminal velocity.
Examiner tips
- Use the term ‘resistive forces’ and state that they increase with speed. Show the balance of forces at constant velocity. Mention Newton’s First Law to explain zero acceleration. Finish with the conclusion of a terminal velocity.
Common mistakes
- Failing to mention that resistive forces increase with speed. Confusing the engine force with the net force. Omitting Newton’s First Law or the concept of zero resultant force.
Mark scheme (4 marks)
- As the train speeds up, resistive forces (such as friction/air resistance/drag) increase
- At constant velocity, the driving force equals the resistive forces (balanced forces / resultant force is zero)
- Because the resultant force is zero, the train does not accelerate (Newton's First Law)
- The train therefore continues at a constant (terminal) velocity
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →