A rock climber descends a cliff face using a rope. As she descends, she moves at a constant velocity. Explain why the climber moves at a constant velocity, referring to the forces acting on her.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A rock climber of mass 65 kg descends a cliff face at a constant velocity. Friction from the rope and air resistance both act on her as she moves downward.
Model answer (4 marks)
The climber’s weight,
W = mg = 65 kg × 9.8 m s⁻² = 637 N, acts vertically downwards. The frictional force from the rope (and any air resistance) acts upward, opposing the motion. Because the climber moves at a constant velocity, the net force on her is zero – the downward weight is exactly balanced by the upward frictional force. According to Newton’s First Law, a zero resultant force means the climber continues to move at a constant velocity.
W = mg = 65 kg × 9.8 m s⁻² = 637 N, acts vertically downwards. The frictional force from the rope (and any air resistance) acts upward, opposing the motion. Because the climber moves at a constant velocity, the net force on her is zero – the downward weight is exactly balanced by the upward frictional force. According to Newton’s First Law, a zero resultant force means the climber continues to move at a constant velocity.
Examiner tips
- Use the exact force names (weight, friction) and directions. Include the calculation of weight to show understanding. Mention Newton’s First Law explicitly. Keep the answer concise and to the point.
Common mistakes
- Forgetting to state the direction of the frictional force. Assuming the rope provides a pulling force rather than a frictional force. Omitting Newton’s First Law or the zero resultant force condition.
Mark scheme (4 marks)
- Weight acts vertically downwards on the climber
- Friction from the rope (and/or air resistance) acts upwards, opposing her motion
- The resultant force is zero / the forces are balanced
- By Newton's First Law, a zero resultant force means the climber 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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