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
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
Which pair of forces act as a couple on the circular object?
ANSWER: A
- In diagram A, the forces are:
- Equal in size
- In opposite directions
- Perpendicular to the distance between them
- B is incorrect as the forces are in the same direction
- C is incorrect as the forces are different in size
- D is incorrect as the distance between the forces is not perpendicular
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
What is the magnitude of the torque of the couple on the ruler when it is at the position shown?
