The couple does not induce translational motion because the two forces that make up the couple are equal and opposing. When it is applied to a body, the net resultant force is zero. Because the algebraic sum of the moments of the two forces around any point in their plane is not zero, a net torque results and it causes pure rotational motion in the body. The size and direction of a couple’s moment about any point on its plane are both constant. The net torque is equal to the product of one of the forces and the distance between the lines of action of the forces.

𝜏_net = Fd