Final answer:
Releasing the discus decreases the discus thrower's moment of inertia as there is less mass far from his axis of rotation, leading to an increased angular velocity to conserve angular momentum.
Step-by-step explanation:
When a discus thrower rotates with a discus in his hand and then releases it, his moment of inertia decreases. This is because the moment of inertia depends on the distribution of mass around the axis of rotation.
When the thrower releases the discus, the distribution of mass changes as there is less mass far from the axis of rotation, leading to a lower moment of inertia.
Another way to understand this concept is by referring to the conservation of angular momentum, which states that if no external torques act on a system, the angular momentum of the system remains constant.
At the moment of release, assuming no other external torques, the thrower's angular velocity will increase to conserve angular momentum because the system's moment of inertia has decreased.