A shopping trolley is pushed from rest along a flat supermarket car park. The person pushing the trolley applies a constant horizontal force. After several seconds, the trolley reaches a constant (terminal) velocity. Explain why the trolley reaches a constant velocity even though the pushing force remains the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
At first the pushing force exceeds the resistive forces, so a net force acts on the trolley and it accelerates (Newton’s second law). As the trolley speeds up the resistive forces (friction and air resistance) increase. When the resistive forces have risen to equal the pushing force the net force becomes zero, so the trolley no longer accelerates and moves at a constant (terminal) velocity.
Examiner tips
- Show the sequence of force comparison → acceleration → increase in resistive forces → equality of forces → zero net force
- Use terms ‘resistive forces’, ‘net force’, ‘Newton’s second law’, ‘terminal velocity’
Common mistakes
- Forgetting to mention that resistive forces increase with speed
- Confusing ‘terminal velocity’ with ‘maximum velocity’
- Using vague wording such as ‘the trolley slows down’ instead of ‘net force becomes zero’
Mark scheme (5 marks)
- As the trolley speeds up, the resistive forces (friction/air resistance) acting on it increase
- At first, the driving/pushing force is greater than the resistive forces, so there is a resultant force
- The resultant force causes the trolley to accelerate (Newton's second law)
- Eventually the resistive forces increase until they equal the pushing force
- When the two forces are equal, the resultant force is zero, so the trolley moves at constant velocity
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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