AQA GCSE Physics

Revision Notes

5.4.1 Moments

Forces & Rotation

  • As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate
    • A system of forces can also do this
  • An example of a rotation caused by a force is on one side of a pivot (a fixed point that the object can rotate around)
    • This rotation can be clockwise or anticlockwise

pivot-force, IGCSE & GCSE Physics revision notes

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

  • More examples of rotation caused by a force are:
    • A child on a see-saw
    • Turning the handle of a spanner
    • A door opening and closing
  • If two forces act on an object without passing through the same point, then the object can still rotate
    • If the forces are equal and opposite, this is known as a couple

rotation-force, IGCSE & GCSE Physics revision notes

The above forces are balanced, but will still cause the object to rotate clockwise as they don’t act through a common point

The Moment of a Force

  • 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)

moment-of-force, IGCSE & GCSE Physics revision notes

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

  • This is why, for example, the door handle is placed on the opposite side to the hinge
    • This means for a given force, the perpendicular distance from the pivot (the hinge) is larger
    • This creates a larger moment (turning effect) to make it easier to open the door
  • Opening a door with a handle close to the pivot would be much harder, and would require a lot more force

Exam Tip

The units of a moment is Newton metres (N m), but can also be Newton centimetres (N cm) ie. where the distance is measured in cm instead

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

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