Definition of a Force Couple

Steps:

• Recall that a couple is two equal forces acting in opposite directions on parallel but separate lines, producing pure rotation without translation.
• Verify option A: opposite directions ensure the forces cancel linearly.
• Verify option B: different lines create a moment arm for torque.
• Verify option C: equal magnitudes ensure zero resultant force.
• Verify option D: a couple requires zero resultant force, not a non-zero one.

Why D is correct:

• By definition, forces in a couple have equal magnitude and opposite direction, so their vector sum (resultant force) is zero, per Newton's laws of motion.

Why the others are wrong:

• A: Opposite directions are required for the forces to cancel and produce only torque.
• B: Different lines ensure the forces are non-collinear, creating a rotational effect.
• C: Same magnitude is essential for the resultant force to be zero.

Final answer: D