A person stands on a set of bathroom scales inside a lift. The scales show a reading greater than the person's actual weight when the lift begins to move upwards from rest. Explain why the scales show a higher reading than the person's actual weight during this upward acceleration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The lift and the person are accelerating upward.
The upward acceleration requires a net upward force.
The normal force from the scales on the person must therefore be greater than the weight.
The scales read this normal force, not the true weight.
Hence the reading is higher than the person’s actual weight.
The upward acceleration requires a net upward force.
The normal force from the scales on the person must therefore be greater than the weight.
The scales read this normal force, not the true weight.
Hence the reading is higher than the person’s actual weight.
Examiner tips
- Use the word ‘accelerating’ and mention the net upward force. Explain that the normal force exceeds weight. State that the scales measure the normal force. Show the logical chain to the higher reading.
Common mistakes
- Confusing weight with normal force. Saying the scales read the person’s weight. Omitting the need for a net upward force.
Mark scheme (5 marks)
- The person (and lift) is accelerating upwards
- A resultant / net force is needed to cause this upward acceleration
- The normal contact force (reaction force from the scales) acting on the person is greater than the person's weight
- The scales measure the normal contact force / reaction force acting on them, not the person's actual weight
- Because the contact force is greater than the weight, the scales display a higher reading than the person's true weight
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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