A rowing boat moves at a constant velocity across a still lake. The rower then stops rowing and the boat gradually slows down and comes to rest. Explain why the boat travels at a constant velocity whilst the rower is rowing, and why the boat decelerates after the rower stops.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
At constant velocity the force produced by the oars equals the resistive forces (water resistance/drag and friction). The forces are balanced, so the resultant force is zero. By Newton’s first law an object continues at constant velocity when no resultant force acts on it.
When the rower stops, the driving force from the oars is removed but the resistive forces remain. Now the forces are unbalanced, giving a resultant force opposite to the motion. By Newton’s second law this resultant force causes a negative acceleration, so the boat decelerates until it comes to rest.
When the rower stops, the driving force from the oars is removed but the resistive forces remain. Now the forces are unbalanced, giving a resultant force opposite to the motion. By Newton’s second law this resultant force causes a negative acceleration, so the boat decelerates until it comes to rest.
Examiner tips
- Show the equality of driving and resistive forces for constant speed; mention the resultant force is zero.
- Explain that removal of the driving force leaves only resistance, giving a non‑zero resultant force.
- Use Newton’s first law for constant velocity and second law for deceleration.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing constant velocity with constant speed; not stating the forces are balanced.
- Forgetting to mention that the resultant force is zero at constant speed.
- Using vague terms like ‘slows down’ without linking to forces and Newton’s laws.
Mark scheme (5 marks)
- At constant velocity, the driving force from the oars equals the resistive forces (water resistance / drag / friction)
- Balanced / equal forces mean there is no resultant force
- Newton's first law: an object continues at constant velocity when there is no resultant force acting on it
- When the rower stops, the driving force is removed but water resistance / drag still acts on the boat
- There is now a resultant force (in the direction opposing motion) so the boat decelerates / Newton's second law: resultant force causes acceleration / deceleration
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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