CIE AS Physics (9702) 2019-2021

Revision Notes

5.1.2 Turning Effects of Forces

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

Couples

  • A couple is a pair of forces that acts to produce rotation only
  • Unlike moments of a single force, the moment of a couple doesn’t depend on a pivot, only on the perpendicular distance between the two forces
  • A couple consists of a pair of forces that are:
    • Equal in magnitude
    • Opposite in direction
    • Perpendicular to the distance between them

Couples diagram, downloadable AS & A Level Physics revision notes

Diagram of a couple

  • Couples produce a resultant force of zero, so, due to Newton’s Second law (F = ma), the object does not accelerate
  • The size of this turning effect is given by its torque

Worked Example

Worked example - couples, downloadable AS & A Level Physics revision notes

Exam Tip

The forces that make up a couple cannot share the same line of action which is the line through the point at which the force is applied. An example of this is shown in the diagram below

 

Forces with same line of action, downloadable AS & A Level Physics revision notes

Torque

  • The moment of a couple is known as a torque
  • You can calculate the torque of a couple with the following equation

Torque τ (N m) = one of the forces (N) × perpendicular distance between the forces (m)

Worked example – perpendicular distance

Worked example - perp distance, downloadable AS & A Level Physics revision notes

Worked example – Non-perpendicular distance

Worked example - non perp distance, downloadable AS & A Level Physics revision notes

Exam Tip

The forces given might not always be perpendicular to the distance between them. In this case, remember to find the component of the force vector that is perpendicular. You can learn more on how to do this in the ‘Resolving Vectors’ section of ‘Scalars & Vectors’

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