This fundamental law relating torque and change of angular momentum is rendered most useful if we divide the total torque into an internal part and an external part,
Ttotal = Tint + Text .
External torques arise from outside the system; internal torques arise from within the system. A sky-diver falling through the air, for example, is subjected to external torques from the air and internal torques from his or her own muscles. According to excellent experimental evidence—evidence based on the validity of the law of angular-momentum conservation—the total internal torques in fact always add up to zero. This is an important statement about nature. It might be called the rotational equivalent of Newton’s third law. Because of Newton’s third law, which leads directly to the conclusion that internal forces always add up to zero, the momentum of an isolated system is conserved. Similarly, the vanishing of total internal torque leads to the conservation of angular momentum for isolated systems. For central forces—those forces acting along the lines joining particles—Newton’s third law leads to the vanishing of total internal torque as well as total internal force. In general, however, the requirement that total internal torque vanishes is a more powerful restriction on the forces of nature than is Newton’s third law.