Physics
Rotational Inertia
Quick fact
Rotational inertia is the rotational analog of mass; it determines how difficult it is to change an object’s spinning motion.
Why this is interesting
Have you ever noticed how a figure skater spins faster when they pull their arms in? It’s not magic—it’s rotational inertia at work.
Read the full explanation
Understanding Rotational Inertia
Imagine holding a bicycle wheel by its axle and trying to spin it. The harder it is to start or stop the wheel, the greater its rotational inertia. This resistance depends on two things: the total mass of the object and how far that mass is from the axis of rotation.
A deeper explanation
Rotational inertia, also called the moment of inertia, describes an object’s resistance to changes in its rotational motion. The more mass an object has, or the farther away that mass is from the axis around which it rotates, the greater its rotational inertia. This principle explains why a gymnast tucks their body during a spin to increase their rotation speed—by reducing their moment of inertia.