Moments (Edexcel IGCSE Physics)

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