Moments (Oxford AQA IGCSE Physics)

Moments

What is a moment?

• Forces exerted on objects can have a turning effect

• The turning effect of a force is called the moment

• Rotation occurs about a fixed pivot

  • Rotation can be clockwise or anticlockwise

Clockwise and anticlockwise rotation

• Examples of the rotation caused by 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

Calculating moments

• The size of a moment is defined by the equation:

M = F × d

• Where:

  • M = moment in newton metres (N m)

  • F = force in newtons (N)

  • d = distance perpendicular to the direction of the force 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 moments on a spanner

• The larger the distance, the less force is required and vice versa

  • 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