Couples Produce Pure Rotation

Steps:

• A couple consists of two equal and opposite parallel forces acting at different points on a rigid body.
• The net force is zero since the forces cancel each other out.
• These forces create a torque, calculated as force magnitude times perpendicular distance between lines of action.
• Torque causes angular acceleration (rotation) without linear acceleration (translation).

Why B is correct:

• By definition in rigid body mechanics, a couple generates torque (rotation) with zero resultant force, per the torque formula $\tau = F \times d$.

Why the others are wrong:

• A: Incorrect; no net force means no translational motion, only rotation.
• C: Incorrect; zero net force prevents translational motion.
• D: Incorrect; torque ensures rotation occurs.

Final answer: B