IGCSE Physics / The turning effect of forces
Moments revision notes
Edexcel International GCSE Physics (4PH1), chapter 5: The turning effect of forces, specification point 1.30P, 1.2P. Free lesson.
Watch the video lesson · IGCSE Physics past papers
A moment is the turning effect of a force about a pivot. You can calculate it using the force and the perpendicular distance from the pivot, then compare clockwise and anticlockwise moments to work out which way an object turns.
What a moment measures
A moment is the turning effect of a force about a pivot, the fixed point an object turns around. The equation is moment = F × d. The distance d is measured at right angles from the pivot to the line along which the force acts. F is force, measured in newtons (N). d is the perpendicular distance, measured in metres (m). Moment is measured in newton metres (N m). The equation is on the equation list.
Where moments act
Many everyday objects turn about a pivot. For the same force, the greater the distance from the pivot, the greater the moment. This is why handles are often placed far from the point an object turns around. A spanner turns about the nut. Your hand applies the force at the end of the handle. A door turns about its hinges. You push on the handle. A see-saw turns about its middle support. The force is each person's weight. A wheelbarrow turns about its wheel. You lift on the handles.
How to calculate a moment
First check that the distance is perpendicular to the force. Convert centimetres to metres before calculating if your answer needs to be in N m. Then substitute the force and distance into moment = F × d.
Why the perpendicular distance matters
If a force acts at an angle, use the shortest, right-angled distance from the pivot to the force's line of action. This is the line along which the force acts. A force at right angles to the object gives the full distance to the pivot, so it can produce a larger moment. For the door example, the handle is 0.80 m from the hinge and the 20 N push is at 30° to the door. The perpendicular distance to the force's line of action is 0.4 m, so the moment is 20 × 0.4 = 8 N m. If the same 20 N force pushes at right angles, the perpendicular distance is 0.80 m. The moment is 20 × 0.80 = 16 N m.
How to tell which way an object turns
A moment can turn an object clockwise or anticlockwise. When moments act in opposite directions, the bigger moment determines which way the object turns. On the see-saw, child A's 300 N weight acts 1.5 m from the pivot, giving an anticlockwise moment of 300 × 1.5 = 450 N m. Child B's 400 N weight acts 1.5 m from the pivot, giving a clockwise moment of 400 × 1.5 = 600 N m. The clockwise moment is bigger, so the see-saw turns clockwise and B goes down.