The conservation law of angular momentum dL T= when M,=0 =const dt The angular momentum is a constant,provided the net torque on the rigid body is zero. J2而=J (M=0)The conservation law of angular momentum dt dLz z    when Mz  0  L const z   The angular momentum is a constant, provided the net torque on the rigid body is zero. 2 2 1 1   J  J (  0) Mz 