Moments (O-Level/IGCSE)

What these notes cover

These notes cover moments for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): the moment of a force about a pivot, the principle of moments, the conditions for equilibrium, centre of gravity and how to find it for a flat object, and stability. They include worked examples of calculating clockwise and anticlockwise moments.

Key formulas and definitions

• A moment is the turning effect of a force: moment = force × perpendicular distance from the pivot to the line of action of the force. Unit: N m.
• Principle of moments: for an object in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point.
• An object is in equilibrium when there is no resultant force and no resultant moment on it.
• Centre of gravity: the point through which all of an object’s weight can be considered to act.
• To find the centre of gravity of an irregular flat lamina, hang it freely from two different points and mark the vertical line from a plumb line each time; the centre of gravity is where the lines cross.
• An object is more stable with a low centre of gravity and a wide base. It topples when the vertical line through its centre of gravity falls outside its base.
• In moment calculations, forces must be in newtons: convert a mass to a weight using W = mg.