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

  • Further examples 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 = Fd

  • 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 far away from 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 will be much harder, and would require a lot more force

Exam Tip

The units of moment is Nm, but can also be Ncm (where the distance is measured in cm instead). If the exam question doesn’t ask for a specific unit, always convert the distance in metres

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