A mass-spring system is set into oscillation. The system is then damped by attaching a card to the mass so that it moves through air. Explain the effect of damping on the amplitude and period of the oscillations.

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).

Model answer (5 marks)

Damping reduces the amplitude of the oscillations as the system loses energy to the surroundings. The energy lost each cycle is dissipated as heat by the damping force (air resistance). The period of the oscillations remains essentially unchanged, so the frequency is almost the same. With enough damping the amplitude eventually falls to zero and the mass comes to rest at the equilibrium position.

Examiner tips

  • Use the word "reduce" or "decrease" for amplitude, not "increase". Mention that energy is lost to heat. State that the period is unchanged. Show the final state: motion stops at equilibrium.

Common mistakes

  • Saying the period increases or decreases. Claiming the energy is lost to work on the mass. Forgetting that the amplitude decreases over time.

Mark scheme (5 marks)

  1. Damping causes the amplitude to decrease over time
  2. Energy is removed from the system (by the damping force / air resistance)
  3. The energy lost per cycle is transferred to the thermal energy store of the surroundings / converted to heat
  4. The period (or frequency) of oscillation remains approximately the same / is unchanged
  5. With sufficient damping the oscillations will stop (the system comes to rest at equilibrium)

Key terms in this question

damping · amplitude · period · oscillation

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