CIE AS Physics (9702) exams from 2022

Revision Notes

4.1.2 Moments

What is a Moment?

  • A moment is the turning effect of a force
  • Moments occur when forces cause objects to rotate about some pivot
  • The moment of a force is given by

               Moment (N m) = Force (N) × perpendicular distance from the pivot (m)

  • The SI unit for the moment is Newton metres (N m). This may also be Newton centimetres (N cm) depending on the units given for the distance

Perpendicular-distance, downloadable AS & A Level Physics revision notes

The force might not always be perpendicular to the distance

 

  • An example of moments in everyday life is opening a door. The door handle is placed on the other side of the door to the hinge (the pivot) to maximise the distance for a given force and therefore a greater moment (turning force). This makes it easier to push or pull it

 

Worked example - mass on ruler, downloadable AS & A Level Physics revision notes

Exam Tip

If not already given, drawing all the forces on an object in the diagram will help you see which ones are perpendicular to the distance from the pivot. Not all the forces will provide a turning effect and it is not unusual for a question to provide more forces than required

The Principle of Moments

  • The principle of moments states: For a system to be balanced (in equilibrium), the sum of clockwise moments about a point must be equal to the sum of anticlockwise moments (about the same point)

Principle of moments, downloadable AS & A Level 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 × d2 = F1 × d1 + F3 × d3

Worked example - principle of moments (1), downloadable AS & A Level Physics revision notes

Worked example - principle of moments (2), downloadable AS & A Level Physics revision notes

Exam Tip

Make sure that all the distances are in the same units and you’re considering the correct forces as clockwise or anticlockwise, as seen in the diagram below

 

Clockwise or anticlockwise moment, downloadable AS & A Level Physics revision notes

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