Q.A couple produces :

A couple is a pair of forces that are equal in magnitude, opposite in direction, and whose lines of action are separated by some nonzero perpendicular distance (so the two forces do not act along the same line). Because the two forces are equal and opposite, their vector sum is exactly zero, so a couple produces zero net force — a body acted on only by a couple stays in translational equilibrium (its center of mass does not accelerate) — yet the two forces do not cancel in their turning effect. Since both forces are on the same side of the body's center relative to their own line of action, their individual moments about the center add together rather than cancelling, producing a net torque, and hence pure rotation with no accompanying translation. …