An object in circular motion is considered to be accelerating even when its speed is constant because acceleration is defined as a change in velocity. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. When an object moves in a circle, even if its speed remains unchanged, its direction is constantly changing as it follows the circular path.
This change in direction means that the velocity of the object is also changing, making it subject to acceleration. In circular motion, this acceleration is referred to as centripetal acceleration, which always points towards the center of the circle. It is this continuous change in direction that qualifies the object as being in a state of acceleration, even when its speed is stable. Therefore, maintaining a constant speed while moving in a curve means the object is always accelerating due to the ongoing alteration in its direction.