Final answer:
Extending the arms increases the skater's moment of inertia and decreases her rotational speed due to conservation of angular momentum.
Step-by-step explanation:
When an ice skater extends her arms horizontally while spinning, it has the effect of decreasing her rotational speed. This is due to the conservation of angular momentum, which states that if no external torque acts on a system, the angular momentum will be conserved. When the skater extends her arms, the distance from her mass to her axis of rotation increases, leading to an increase in her moment of inertia. To conserve angular momentum, her rotational speed must decrease. Conversely, when she pulls her arms in, her moment of inertia decreases, and her rotational speed increases.