<Dd> τ (\ displaystyle \ tau) is the moment of couple </Dd> <Dd> F is the magnitude of one of the forces </Dd> <Dd> d is the perpendicular distance between the forces, sometimes called the arm of the couple </Dd> <P> The magnitude of the torque is always equal to F d, with the direction of the torque given by the unit vector e ^ (\ displaystyle (\ hat (e))), which is perpendicular to the plane containing the two forces . When d is taken as a vector between the points of action of the forces, then the couple is the cross product of d and F, i.e. </P>

Which pair of forces acts only as a couple on the circular object