Forces in a Couple Produce Pure Rotation

Steps:

• Recall that a couple consists of two equal forces acting in opposite directions on parallel lines.
• Calculate the resultant force: since forces are equal and opposite, their vector sum is zero.
• Assess the moment: the forces create a torque about any point, but no net force translates.
• Confirm the defining property: zero resultant force with nonzero moment distinguishes a couple.

Why C is correct:

• By definition, the two equal and opposite forces in a couple cancel each other, yielding zero resultant force per Newton's laws of motion.

Why the others are wrong:

• A: Forces in a couple act in opposite directions, not the same.
• B: Forces act on different lines of action, not through the same point.
• D: A couple produces a nonzero resultant moment (torque), causing rotation.

Final answer: C