A car suspension system uses springs and shock absorbers (dampers) to reduce the vibrations felt by passengers when driving over bumps. Without the dampers, the suspension would undergo free oscillations after hitting a bump. Explain how the dampers reduce the amplitude of these oscillations and why engineers choose a level of damping that allows the car body to return to its rest position as quickly as possible without oscillating.

AQA A-Level Physics (7408) — 3.6.1 Periodic Motion · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Without dampers, a car suspension spring would continue to oscillate for many cycles after hitting a single bump in the road.

Model answer (5 marks)

The dampers perform work against a resistive (viscous) force, removing kinetic and potential energy from the suspension. Each time the car body passes through the equilibrium position the damper does work, so the energy of the next oscillation is lower. Consequently the amplitude of successive oscillations decreases progressively. The suspension has a natural frequency – the frequency it would oscillate at if no damping were present. Engineers select a damping level close to the critical value: heavy damping would make the body return to equilibrium very slowly, while light damping would allow many oscillations before settling. Critical damping gives the fastest return to the rest position without overshoot, so the chosen damping is a compromise that minimises vibration while avoiding resonance or excessive oscillation.

Examiner tips

  • Use the term ‘viscous damping’ or ‘resistive force’ to show understanding of energy removal.
  • Explain the effect on amplitude over successive cycles.
  • Mention natural frequency and its relevance to damping choice.
  • State that critical damping gives quickest return without oscillation.

Mark scheme (5 marks)

  1. The dampers remove/transfer energy from the oscillating system (accept: do work against a resistive/friction force)
  2. Each successive oscillation has a smaller amplitude because energy has been removed (amplitude decreases over time)
  3. The natural frequency of the suspension is the frequency at which it would oscillate freely / without damping
  4. The chosen level of damping is critical / heavy damping means the system returns to equilibrium slowly whereas light damping means many oscillations occur before returning to equilibrium (credit: identifying the need to avoid resonance / excessive oscillation)
  5. The optimum (critical) damping returns the system to equilibrium in the shortest time without oscillating / a compromise between light and heavy damping is chosen

Key terms in this question

damping · amplitude · free oscillation

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