Q.Define couple.

A couple is a pair of equal, opposite, parallel forces with different lines of action; since the net force is zero, it produces a pure turning effect (torque) with no translation.

Consider two forces of equal magnitude F acting in opposite directions along two parallel lines separated by a perpendicular distance d.

Net force = F + (-F) = 0, so the centre of mass does not accelerate linearly — there is no translational effect.

…

A couple consists of two equal-magnitude, oppositely-directed forces whose lines of action are parallel but not the same line. Their resultant force is zero (no translation), but because they act along different lines, their torques about any point add up rather than cancel, producing pure rotation.

Consider two forces F and -F applied at two different points separated by a perpendicular distance d (the arm of the couple).

Net force = F + (-F) = 0, so the centre of mass of the body does not accelerate — there is no translational effect.

…