system
Given the following post, respond as if you're commenting on Reddit:
human
Rotation inertia question: why doesn't the *change* in direction of each portion of an object, needed to keep the object spinning, constitute an acceleration, requiring work?  

I am sure this is a boneheaded question but my meager physics understanding leads me to a bit of a perceived contradiction.
gpt
Even though every point on a rotating object experiences an acceleration towards the object's center of rotation, the object does not gain/lose energy because the force vector and velocity vector of each point are perpendicular. Basically, the force isn't contributing at all in the direction of motion, so no energy is added or lost.