• A moment is the turning effect of a force
• Moments occur when forces cause objects to rotate about some pivot
• The moment of a force is given by

               Moment (N m) = Force (N) × perpendicular distance from the pivot (m)

• The SI unit for the moment is Newton metres (N m). This may also be Newton centimetres (N cm) depending on the units given for the distance

The force might not always be perpendicular to the distance

• An example of moments in everyday life is opening a door. The door handle is placed on the other side of the door to the hinge (the pivot) to maximise the distance for a given force and therefore a greater moment (turning force). This makes it easier to push or pull it

• A couple is a pair of forces that acts to produce rotation only
• Unlike moments of a single force, the moment of a couple doesn’t depend on a pivot, only on the perpendicular distance between the two forces
• A couple consists of a pair of forces that are:
  • Equal in magnitude
  • Opposite in direction
  • Perpendicular to the distance between them

Diagram of a couple

• Couples produce a resultant force of zero, so, due to Newton’s Second law (F = ma), the object does not accelerate
• The size of this turning effect is given by its torque

Worked Example

• The moment of a couple is known as a torque
• You can calculate the torque of a couple with the following equation

Torque τ (N m) = one of the forces (N) × perpendicular distance between the forces (m)

Worked example – perpendicular distance

Worked example – Non-perpendicular distance