A cargo ship is travelling across the ocean at a constant velocity. The ship's engines produce a driving force. Explain why the ship travels at a constant velocity rather than accelerating.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A large cargo ship cruises at a steady speed across the ocean. Its engines are running and producing a continuous driving force forwards. Despite this, the ship does not speed up.
Model answer (4 marks)
The engines produce a driving force that pushes the ship forward.
The ship also experiences a resistive force (water resistance/drag) that acts in the opposite direction.
These two forces are equal in magnitude, so the net force on the ship is zero.
With a resultant force of zero, Newton’s First Law tells us the ship continues at a constant velocity.
The ship also experiences a resistive force (water resistance/drag) that acts in the opposite direction.
These two forces are equal in magnitude, so the net force on the ship is zero.
With a resultant force of zero, Newton’s First Law tells us the ship continues at a constant velocity.
Examiner tips
- Show the two forces and state they are equal and opposite; mention Newton’s First Law explicitly.
- Use the exact terms ‘driving force’, ‘resistive force’, ‘net force’, ‘zero’, and ‘constant velocity’.
- Keep the answer concise – 4 points can be covered in 4 short sentences.
Common mistakes
- Forgetting to mention the resistive force or that it is opposite to the driving force.
- Using vague terms like ‘friction’ without specifying it is water resistance.
- Not stating that the net force is zero or that Newton’s First Law applies.
Mark scheme (4 marks)
- The ship experiences a resistive force (friction / drag / water resistance) acting in the opposite direction to motion
- The driving force and the resistive force are equal in magnitude / balanced
- The resultant force on the ship is zero
- According to Newton's First Law, an object with a resultant force of zero continues at a constant 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 →
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