A tall building sways in the wind. Engineers design the building so that its natural frequency of oscillation is very different from the typical frequency of wind gusts. Explain why this design feature is important, and describe what would happen if the two frequencies were equal.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A building has a natural frequency at which it will oscillate freely when undamped. Wind gusts act as a periodic driving force, giving the building a driving frequency. If the driving frequency matches the natural frequency, resonance occurs. At resonance the amplitude of oscillation increases greatly, so the building would oscillate with maximum amplitude. This could cause structural damage or collapse, so keeping the frequencies different prevents resonance.
Examiner tips
- Use the term ‘natural frequency’ and ‘driving frequency’.
- Explain that resonance leads to large amplitude and potential damage.
- Show the cause–effect chain: matching frequencies → resonance → large amplitude → damage.
- Use ‘prevent resonance’ as the design goal.
Common mistakes
- Confusing natural frequency with damping or ignoring the driving force.
- Saying the building will ‘stop’ or ‘not move’ instead of ‘oscillate with large amplitude’.
- Omitting the consequence of resonance such as structural damage.
Mark scheme (5 marks)
- The building has a natural frequency at which it oscillates freely / when undamped
- Wind gusts act as a periodic driving force / the wind applies a driving frequency to the building
- If the driving frequency equals the natural frequency, resonance occurs
- At resonance the amplitude of oscillation increases greatly / the building oscillates with maximum amplitude
- This could cause structural damage to / collapse of the building, so keeping frequencies different prevents resonance
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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