The net torque associated with all internal forces, however, is zero. Recall
that Newton’s third law tells us that internal forces between particles of the system
occur in pairs that are equal in magnitude and opposite in direction. If we assume
these forces lie along the line of separation of each pair of particles, the total
torque around some axis passing through an origin O due to each action–reaction
force pair is zero (that is, the moment arm d from O to the line of action of the
forces is equal for both particles, and the forces are in opposite directions). In the
summation, therefore, the net internal torque is zero. We conclude that the total
angular momentum of a system can vary with time only if a net external torque is
acting on the system