Today, we call this observation the law of conservation of momentum . The momentum of an object is the product of its mass and its velocity. There is an equivalent law for rotating objects. A rotating object tends to remain rotating with a constant angular momentum unless it is acted upon by an outside twisting force. The definition of angular momentum is slightly more complex than that of linear momentum.
Angular momentum is the product of two quantities known as angular velocity and moment of inertia .
Moment of inertia depends on both the mass of an object and on how that mass is distributed. The farther from the axis of rotation the mass is located, the larger the moment of inertia. So your moment of inertia is smaller when your arms are held at your sides and larger when your arms are extended straight out. If the motion of a rotating system is not affected by an outside twisting force, then angular momentum is conserved for this system, which means that the angular momentum stays the same. A person sitting on a rotating chair or stool approximates a system in which angular momentum is conserved.