The quantity that is always changing for an object moving in a circular path is its velocity.

Here's why:

* Velocity is a vector: It has both magnitude (speed) and direction.

* Constant speed, changing direction: While an object moving in a circle might maintain a constant speed, its direction is constantly changing.

* Tangential velocity: The object's velocity is always tangent to the circular path, meaning it's always changing as the object moves along the curve.

Let me know if you'd like more explanation on any of these concepts!