Which statement describes a pure moment produced by a couple?

Which statement describes a pure moment produced by a couple?

• A. Produces a net force but no moment
• B. Lines of action pass through a single point
• C. Requires a third force to maintain equilibrium
• D. Produces a pure moment without a resultant force

Answer: (D)

A pure moment produced by a couple arises when two equal and opposite forces act with parallel, non-collinear lines of action. Their sum is zero, so there’s no net force, but they create a turning effect or moment that tends to rotate the body. That moment has a magnitude equal to the force magnitude times the perpendicular distance between the lines of action, and it is the same regardless of the point about which you take moments. This is exactly what the statement describes: a pure moment without a resultant force. The other ideas describe different force systems—having a net force with no moment would be like a single force; lines of action meeting at a point imply concurrent forces that produce a resultant force; and requiring a third force to maintain equilibrium is not what defines a couple.

A pure moment produced by a couple arises when two equal and opposite forces act with parallel, non-collinear lines of action. Their sum is zero, so there’s no net force, but they create a turning effect or moment that tends to rotate the body. That moment has a magnitude equal to the force magnitude times the perpendicular distance between the lines of action, and it is the same regardless of the point about which you take moments.

This is exactly what the statement describes: a pure moment without a resultant force. The other ideas describe different force systems—having a net force with no moment would be like a single force; lines of action meeting at a point imply concurrent forces that produce a resultant force; and requiring a third force to maintain equilibrium is not what defines a couple.