Explain why an object moving in
a circle is accelerating, even if it
moves with a constant speed

An object moving in  a circle is accelerating, even if it  moves with a constant speed because the velocity changes.

Explanation:

The direction of an object changes constantly during its movement in circle at a constant speed. As the direction changes it results in changing the velocity of that object. This happens since velocity  has direction and magnitude.as it is vector quantity.

An object will have acceleration only if a resultant force is acting on that body. When an object is moving in a circle, this resultant force is the centripetal force that is acting towards the center of the circle.