CIE IGCSE Physics

Revision Notes

1.5.5 Turning Effect

The Moment of a Force

  • A moment is the turning effect of a force
  • Moments occur when forces cause objects to rotate about some pivot
  • The size of the moment depends upon:
    • The size of the force
    • The distance between the force and the pivot
  • The moment of a force is given by the equation:
Moment = Force × perpendicular distance from the pivot
  • Moments have the units newton centimetres (N cm) or newton metres (N m), depending on whether the distance is measured in metres or centimetres

Toppling block, IGCSE & GCSE Physics revision notes

Diagram showing the moment of a force causing a block to topple

  • Some other examples involving moments include:
    • Using a crowbar to prize open something
    • Turning a tap on or off
    • Opening or closing a door

The Principle of Moments

  • The principle of moments states that:
    • For a system to be balanced, the sum of clockwise moments must be equal to the sum of anticlockwise moments

 

balanced-seesaw, IGCSE & GCSE Physics revision notes

Diagram showing the moments acting on a balanced beam 

  • In the above diagram:
    • Force F2 is supplying a clockwise moment;
    • Forces F1 and F3 are supplying anticlockwise moments
  • Hence:

F2 x d2 = F1 x d1 + F3 x d3

 

Extended Only

Example of The Principle of Moments

  • The principle of moments doesn’t just apply to seesaws – it is important in many other situations as well such as, for example, a shelf:

 

Moments worked example, IGCSE & GCSE Physics revision notesTo prevent the shelf from collapsing, the support must provide an upward moment equal to the downward moment of the vase

 

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