Couples
- A couple is a pair of forces that acts to produce rotation only
- The moment of a single force depends on the perpendicular distance to the pivot
- However, the moment of a couple depends 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
A pair of forces, or couple
The moment of a couple depends on the perpendicular distance between the two forces
- Couples produce a resultant force of zero
- Therefore, due to Newton’s Second law of motion, F = ma, the object does not accelerate
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
Examiner Tip
The forces that make up a couple cannot share the same line of action. The line of action is a line through the point at which the force is applied.