Force Couple on a Rod

Steps:

• Recall a couple is two equal, opposite, parallel forces separated by a distance, producing pure rotation.
• Identify diagrams with two forces each of magnitude F on the rod.
• Verify forces are parallel, opposite in direction, and not collinear to create torque without net force.
• Confirm the diagram shows no translational motion, only rotational tendency.

Why D is correct:

• D depicts two equal forces F acting parallel and oppositely at opposite ends of the rod, perpendicular to the line joining them, satisfying the couple definition where torque τ = F × d (d = perpendicular distance).

Why the others are wrong:

• A: Forces act in the same direction, causing net translation, not a couple.
• B: Forces are not parallel or opposite, resulting in unbalanced motion.
• C: Forces are collinear and opposite, producing no torque, just compression or tension.

Final answer: D