An isolated object is initially spinning at a constant speed. Then, although no external forces act upon it, its rotational speed increases. This must be due to
A) an increase in the moment of inertia.
B) an increase in the mass.
C) an increase in the angular momentum.
D) a decrease in the moment of inertia.
E) impossible, angular momentum conservation is violated.

Question

Quizplus · Accepted Answer

When an isolated system experiences an increase in rotational speed without the influence of external forces, it implies a decrease in the moment of inertia. This is in accordance with the conservation of angular momentum, which states that if no external torque acts on a system, the total angular momentum of the system remains constant. Therefore, if the rotational speed increases, the moment of inertia must decrease to keep the product of the moment of inertia and angular velocity (angular momentum) constant.

An Isolated Object Is Initially Spinning at a Constant Speed