Principle of Moments. Balance it.

1. Find the centre of gravity of the metre rule by balancing it on the knife-edge pivot without any masses; record the pivot position (typically the 50 cm mark).
2. Suspend a known mass m₁ at a measured distance d₁ to the left of the pivot using a thin loop of thread.
3. Suspend a second known mass m₂ to the right of the pivot.
4. Adjust the position of m₂ until the rule rests horizontally. Measure d₂ from the pivot.
5. Calculate forces F₁ = m₁·g and F₂ = m₂·g.
6. Test the balance condition: F₁ · d₁ = F₂ · d₂.
7. Repeat for at least 3 different combinations of mass and distance.

Analytical control: the two products should agree within the limits of experimental accuracy, confirming the Principle of Moments.

• Rule's own weight — first locate the rule's own centre of gravity so its weight does not produce an unaccounted moment.
• Thread loops not vertical — ensure loops are perpendicular to the rule so distances are measured correctly.
• Pivot friction — use a sharp knife-edge pivot; gently tap the rule to overcome stiction before reading.
• Rule sliding on pivot — re-centre between trials.
• Reading position — measure from the pivot (not from the rule's zero) to avoid arithmetic errors.

• Uniform metre rule (mass uniformly distributed)
• Triangular knife-edge pivot (or pencil/rod)
• Set of slotted masses (100 g, 200 g, etc.)
• Thin string loops (or paper clips) to hang the masses
• Spirit level (to verify horizontal)

• 1.5.3 Turning effect of forces — describe the moment of a force as a measure of its turning effect. Recall and use moment = force × perpendicular distance.
• 1.5.4 Conditions for equilibrium — state the principle of moments: when in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments. Apply to a single pivot.
• 1.5.4 — perform and describe an experiment for an object in equilibrium with several forces acting.
• Paper 6 — describe one source of error and a corresponding precaution; verify a stated relationship by comparing two calculated quantities.