4.1.3—A couple is a pair of forces that acts to produce rotation only
- Syllabus
- 9702–2028–2029
- Objective
- 4.1.3
- Level
- AS
A couple is two equal opposite forces whose separation creates a pure turning effect
A couple consists of two equal, parallel forces acting in opposite directions on different lines. Its resultant force is zero, but it produces a moment.
The turning effect is independent of the chosen pivot. Use the perpendicular separation of the two lines of action and identify the sense of rotation.
Two 15 N forces separated by 0.20 m form a couple with moment 3.0 N m, even though their resultant force is zero.
A pair of equal forces only forms a couple when their lines of action are separated; collinear opposite forces cancel without rotation.
ConceptA-Level CAIE Physics AS