O-Level Physics Centre of Gravity Practical: The Suspended Lamina

When a lamina hangs freely and settles, its centre of gravity lies vertically below the suspension point. One vertical line gives possible positions. Suspending it from another point gives a second line, and their intersection locates the centre of gravity in the lamina's plane.

This supervised method uses a prepared flat lamina with existing holes, an assigned suspension fitting and a plumb line. Cutting or drilling a specimen needs a separate preparation procedure. Read the general lab safety guide before the apparatus briefing.

Why the lines meet

The 2026 O-Level Physics syllabus, 6091, includes centre-of-gravity investigations in its practical scope on pages 31-32. This is an illustrative lamina method, not a prediction of the specimen in an examination.

If the weight's line of action did not pass through the suspension point, it would exert a turning effect and the freely hanging lamina would rotate. At rest, the plumb line identifies the vertical through that point. In an approximately uniform gravitational field, centre of gravity and centre of mass coincide. The measurement still belongs to the actual specimen, including any permanent attachments.

Prepare the suspension

Have the trainer check that the stand is stable, the fitting permits free movement and the supplied plumb weight hangs low with a clear area below. The lamina must not rub against the stand or have its rotation prevented by a tight fitting.

Use only the prepared holes. If a hole is torn, too tight or poorly placed, stop and show the trainer. Do not enlarge it with a pencil, scissors or an improvised sharp tool. Keep fingers clear of the plumb weight and any exposed fitting point.

The marking method should also be assigned in advance. Marking or tracing must not hold the lamina at an arbitrary angle and turn that position into an apparent equilibrium reading.