IGCSE Physics / The turning effect of forces
The principle of moments
Edexcel International GCSE Physics (4PH1), chapter 5: The turning effect of forces, specification point 1.32P. Free lesson.
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The principle of moments explains how forces can balance without an object turning. It lets you find an unknown force or distance by comparing clockwise and anticlockwise moments about the same point.
What the principle of moments says
An object in equilibrium is balanced and not turning. For it to stay that way, the total clockwise turning effect must equal the total anticlockwise turning effect, measured about the same point, usually the pivot. A moment is the turning effect of a force. Calculate it using force × perpendicular distance from the pivot, giving a unit of newton metres (N m). Use the total or sum on each side because several forces can create moments in the same direction. A force acting at the pivot has zero distance from it, so it creates no moment about that pivot.
How to find a missing distance
For a balanced see-saw, set the clockwise moment equal to the anticlockwise moment. A heavier person must sit closer to the pivot if the see-saw is to balance. Choose the pivot as the point to take moments about. Match each force to its distance from the pivot, then set the two totals equal.
How to find a missing force
Mass is not a force, so convert each mass into its weight before calculating moments. Use W = mg, where W is weight in newtons (N), m is mass in kilograms (kg), and g is gravitational field strength in newtons per kilogram (N/kg). Convert distances to metres and measure them from the pivot, not from the end of the rule. Add moments from multiple loads that turn in the same direction. The metre rule's own weight acts at its centre. If its centre is at the pivot, it has no moment about that pivot.
How to test the principle
You can investigate moments with a metre rule, a pivot and slotted masses. When the rule balances, the clockwise and anticlockwise moments should be equal within experimental error. Record each weight in newtons and its distance from the pivot.