Two equal and opposite parallel forces produce pure rotation independent of the reference point.
A couple is a pair of equal, opposite, non-collinear forces; its moment F·d is the same about every point. It can be moved anywhere in the body without changing the effect. Examples: steering wheel turning, a screwdriver.
Two 30 N forces 0.2 m apart give a couple of 6 N·m.
A couple is the simplest way to apply pure rotation, and it explains why steering wheels and wrench handles work as they do.
Two equal and opposite parallel forces produce pure rotation independent of the reference point.
A couple is the simplest way to apply pure rotation, and it explains why steering wheels and wrench handles work as they do.
The two forces produce zero net force, so the moment is the same wherever you take it. Only the rotation matters.
Multiply one of the forces by the perpendicular distance between the two lines of action.
Typical applications include applying torque to a valve handle with two hands, modelling the turning effect of two opposed forces on a shaft and describing the rotation caused by a motor's electromagnetic forces on a coil.
Study "Moment of a force" first (topic 7 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
Net force is zero. Pure rotation effect. Couple moment is a free vector.
Two 30 N forces 0.2 m apart give a couple of 6 N·m.
The next topic is "Equilibrium of rigid bodies", which builds directly on this one.
It is classed as Beginner in Engineering Mechanics (topic 8 of 106). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.
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