CIE AS Physics (9702) exams from 2022

Revision Notes

4.1.3 Turning Effects of Forces

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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