O-Level Physics Mechanics Practical Playbook (Paper 3)

2 | Mechanics investigations that recur in Paper 3

2.1 | Determining the acceleration of free fall (g)

• The syllabus lists a dedicated (g) determination (item 3, SEAB 2026 Physics syllabus). Be ready to linearise $s = 0.5, g, t^{2}$ or use pendulum timing.
• Planning: describe how you measure displacement or length, the number of repeats, and how you mitigate launch height errors.
• MMO: keep release height fixed, use electronic timing where supplied, and note safety (clear fall path).
• PDO / ACE: tabulate (t) and (t^2), plot (s) against (t^2), and justify any gradient-based estimate of (g).

2.2 | Balanced and unbalanced forces

• SEAB expects experiments exploring equilibrium and resultant forces (item 4, SEAB 2026 Physics syllabus). Typical setups include Newton meters with angled strings or trolleys on slopes.
• Planning: cite friction mitigation (smooth runway, light gates) and describe how you vary a single parameter.
• ACE: comment on frictional losses, pulley mass, or string stretch - improvements could include using a ticker timer or data logger for consistent acceleration readings.

2.3 | Principle of moments and centre of gravity

• The principle of moments (item 5) and centre of gravity determinations (item 6) remain staple tasks (SEAB 2026 Physics syllabus).
• MMO: ensure pivot points are sharply defined, measure perpendicular distances, and add weights symmetrically.
• PDO: tabulate clockwise and counterclockwise moments with units (N cm).
• ACE: explain how small angular displacement or weight hook thickness affects torque accuracy, and propose using thinner hooks or mirror scales to reduce parallax.

2.4 | Density of solids and liquids

• Density tasks (item 2) bridge measurement with mechanics because they often precede buoyancy or pressure experiments (SEAB 2026 Physics syllabus).
• Show how you dry irregular solids before volume displacement, state that water temperature should be constant, and mention corrections for air bubbles.
• Present mass–volume tables with consistent units and compute mean densities, flagging outliers or systematic offsets.

3 | Planning and ACE checklists tailored to mechanics

• Refer to all four assessed skill strands - P, MMO, PDO, ACE - when drafting plan responses, because Paper 3 can embed planning elements inside measurement tasks (SEAB 2026 Physics syllabus).
• Quantify uncertainties: quote ruler precision (±0.1 cm), stopwatch reaction time (±0.1 s), and balance resolution (±0.01 g).
• State realistic improvements such as using fiducial markers for motion timing, digital scales with higher resolution, or clamped protractors when measuring angles.
• Follow your school’s lab safety rules - secure masses, keep the area clear of trip hazards, and store weights after use.

4 | Mechanics practice prompts between lab sessions

1. (g) plotting drill: Collect pendulum times for lengths 40–90 cm, plot (T^2) vs (L), and extract (g) from the gradient; write an ACE paragraph explaining how timing jitter affects the slope.
2. Moments investigation: Build a metre-rule balance with unequal masses, measure arm lengths, and compute clockwise vs counterclockwise moments; discuss how hook thickness changes lever-arm measurements.
3. Density audit: Measure three irregular metal pieces, record mass, displaced water volume, and compute density; explain deviations from handbook values by referencing entrapped bubbles or misread menisci.

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Sources

1. SEAB, GCE O-Level Physics (6091) Syllabus 2026

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