Which factors influence moment of inertia?

• A. The surface area of contact
• B. Air resistance and gravity
• C. Position, size, speed of the performer
• D. Temperature and humidity

Answer: (C)

Moment of inertia tells us how hard it is to change rotational motion based on how mass is distributed around the axis of rotation. In a performer’s body, where mass sits and how far it is from the rotational axis are the key factors. If the body stretches limbs away from the axis or swells with larger overall size, more mass sits farther from the center, increasing inertia and making it harder to start or speed up spinning. Conversely, bringing limbs closer to the body or reducing overall size moves mass toward the axis, lowering inertia and allowing easier rotation.

Speed of the motion is included here because rapid movements often involve changing body configuration—tucking in or extending limbs—that alter how far mass is from the axis. While the inertia itself is determined by mass distribution, the visible effect in performance comes from how quickly those changes are made and how they reposition the mass relative to the axis.

Surface area of contact, air resistance and gravity, and environmental factors like temperature or humidity don’t determine how mass is distributed around the axis, so they don’t influence moment of inertia in the same way.