Moments (Edexcel IGCSE Physics)

The moment of a force

Spec point — 1.30P · know and use the relationship between the moment of a force and its perpendicular distance from the pivot: moment = force × perpendicular distance from the pivot (physics only)

The moment of a force

• The moment of a force is the turning effect produced when a force is exerted on an object

• Examples of the rotation caused by the moment of a force are:

  • A child on a see-saw
  • Turning the handle of a spanner
  • A door opening and closing
  • Using a crane to move building supplies
  • Using a screwdriver to open a tin of paint
  • Turning a tap on and off
  • Picking up a wheelbarrow
  • Using scissors

• Forces can cause the rotation of an object about a fixed pivot

• This rotation can be clockwise or anticlockwise

Clockwise and anti-clockwise rotation

Consider the hands of a clock when deciding if an object will rotate in a clockwise or anti-clockwise direction

• A force applied on one side of the pivot will cause the object to rotate

An object rotating clockwise about a pivot

The force applied will cause the object to rotate clockwise about the pivot

• A moment is defined as: The turning effect of a force about a pivot
• The size of a moment is defined by the equation:

M = F × d

• Where:

  • M = moment in newton metres (Nm)
  • F = force in newtons (N)
  • d = perpendicular distance of the force to the pivot in metres (m)

• The forces should be perpendicular to the distance from the pivot

  • For example, on a horizontal beam, the forces which will cause a moment are those directed upwards or downwards

The moment of a force exerted on a spanner

The moment depends on the force and perpendicular distance to the pivot

• Increasing the distance a force is applied from a pivot decreases the force required

  • If you try to push open a door right next to the hinge it is very difficult, as it requires a lot of force
  • If you push the door open at the side furthest from the hinge then it is much easier, as less force is required

Forces required to open a door

A greater force is required to push open a door next to the hinges than at the door handle

Exam Hint The moment of a force is measured in newton metres (N m), but can also be newton centimetres (N cm) if the distance is measured in cm instead.

If your IGCSE exam question doesn't ask for a specific unit, always convert the distance into metres