# Explain why a person standing on a bathroom scale in a lift (elevator) that is accelerating downwards reads a value less than their true weight.

> IB DP Physics Standard Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks

> A person stands on a bathroom scale inside a lift. The lift begins to accelerate downwards from rest.

## Mark scheme (4 marks)

1. Weight (gravitational force) acts downward on the person at all times
2. The scale reads the normal/contact force that it exerts upward on the person (Newton's third law pair / reaction force)
3. Applying Newton's second law: the net downward force provides the downward acceleration, so the normal force N must be less than the weight mg (i.e. mg − N = ma)
4. Because the contact/normal force is less than the weight, the scale displays a value smaller than the person's true weight

## Key terms

- [weight](https://www.gradenine.co.uk/glossary/weight)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-person-standing-on-cc6fb377) · Published by Druglandscape Ltd.