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Physics · 1.5.4 Centre of gravity · Paper 6 practical

Centre of Mass. Find it.

Locate the centre of gravity of an irregular flat lamina using the plumb-line method. Hang the lamina from three different points; the three plumb lines meet at the centre of gravity.

0625 Topic 1.5.4 — Centre of gravity Paper 6 — ATP
Setup — pick a hole (A, B or C), wait for the lamina to settle, then press Mark plumb line.

Shortcuts 1/2/3 select hole A/B/C · Space mark line · R reset.

Choose suspension

—

Plumb lines drawn

From hole A
— not yet
From hole B
— not yet
From hole C
— not yet
Estimated centre of gravity
requires 3 lines
The plumb line from each suspension point passes through the centre of gravity. Three lines should intersect at one point.
📋 Method (Cambridge ATP procedure)
  1. Punch three small holes (A, B, C) near the outer edge of the cardboard lamina, spaced well apart.
  2. Suspend the lamina from hole A on a smooth, horizontally clamped pin. Allow it to swing freely with minimal friction until it comes to rest.
  3. Hang a plumb line from the pin so the string drapes vertically in front of the cardboard.
  4. Once the lamina and plumb line have stopped moving, mark two or three dots on the cardboard directly behind the string.
  5. Remove the lamina and draw a straight line through the dots using a ruler.
  6. Repeat steps 2–5 with the lamina suspended from hole B, then from hole C.
  7. The unique point where all three lines intersect is the centre of gravity.

Test: the lamina should balance on a fingertip placed under this intersection point.

⚠ Sources of error & precautions
  • Pin friction — use a smooth, thin pin so the lamina swings freely; otherwise it may stop before reaching equilibrium.
  • Plumb line still moving — wait until both the lamina and the plumb line have completely stopped before marking dots.
  • Holes far apart — choose holes well separated near the edge so the three lines cross at large angles, reducing geometric uncertainty in the intersection.
  • Pencil dot accuracy — use a sharp pencil and mark behind the string, viewed perpendicular to the lamina.
  • Air drafts — perform away from open windows or fans.
🧪 Apparatus list
  • Irregular cardboard lamina
  • Retort stand with horizontal clamped pin
  • Plumb line — string with a small mass (e.g. plasticine ball) at the end
  • Sharp pencil and ruler
  • Pencil sharpener / awl to make three holes near the edge
🎯 Syllabus reference (0625)
  • 1.5.4 Centre of gravity — define the centre of gravity; describe an experiment to determine the position of the centre of gravity of an irregularly shaped plane lamina; describe qualitatively the effect of the position of the centre of gravity on the stability of simple objects.
  • Paper 6 — set up a practical apparatus; record observations clearly; identify sources of error and improvements.

Ask the lab assistant