A student sets up a simple pendulum and observes it swinging back and forth. Describe the motion of the pendulum bob in terms of its displacement, amplitude, and time period. Include in your answer an explanation of what happens to the time period if the amplitude is made larger.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple pendulum consists of a small bob attached to a string. When pulled to one side and released, it swings repeatedly through its equilibrium (undisturbed) position.
Model answer (5 marks)
The bob oscillates back and forth about its equilibrium (undisturbed) position.
The amplitude is the maximum displacement of the bob from its equilibrium position.
The time period is the time taken for one complete oscillation (swing).
If the amplitude is increased, the time period does not change.
Although the bob travels a greater distance with a larger amplitude, it also moves faster, so the time period remains unchanged.
The amplitude is the maximum displacement of the bob from its equilibrium position.
The time period is the time taken for one complete oscillation (swing).
If the amplitude is increased, the time period does not change.
Although the bob travels a greater distance with a larger amplitude, it also moves faster, so the time period remains unchanged.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "oscillates back and forth about its equilibrium position").
- Include all four required points and the reasoning for the time period change to gain full marks.
- Keep the answer concise – one sentence per point – to fit the 5‑mark limit.
Common mistakes
- Writing the amplitude as the average displacement instead of the maximum displacement.
- Claiming the time period increases with amplitude, which is incorrect for a simple pendulum at small angles.
- Using non‑examiner terminology such as "period of oscillation" instead of "time period".
Mark scheme (5 marks)
- The bob oscillates/moves back and forth about its equilibrium (undisturbed) position
- Amplitude is the maximum displacement of the bob from its equilibrium (undisturbed) position
- Time period is the time taken for one complete oscillation / swing
- Increasing the amplitude does NOT change the time period
- Correct reasoning: although the bob travels a greater distance with larger amplitude, it also moves faster, so the time period is unchanged
Key terms in this question
amplitude · time period · displacement
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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