Total internal torque is zero because the torque resulting from each internal action reaction force pair is zero. Thus ∑n=∑ →dL (10-9) That is: the net external torque acting on a system of particles is equal to the time rate of change of the total angular momentum of the system Note that: (1) the torque and the angular momentum must be calculated with respect to the same origin of an inertia reference frame (2)Eg(10-9) holds for any rigid body.Total internal torque is zero because the torque resulting from each internal action- reaction force pair is zero. Thus (10-9) That is: “the net external torque acting on a system of particles is equal to the time rate of change of the total angular momentum of the system.” Note that: (1) the torque and the angular momentum must be calculated with respect to the same origin of an inertia reference frame. dt d L n ext → → →  =  = (2) Eq(10-9) holds for any rigid body