Here's why:
* Circular motion requires a change in direction. Even if the object's speed is constant, its velocity is constantly changing because its direction is changing.
* Acceleration is a change in velocity. Since velocity involves both speed and direction, a change in either one means there's acceleration.
* Centripetal acceleration is always directed towards the center of the circle. This is what causes the object to constantly change direction and stay in its circular path.
Think of swinging a ball on a string. You feel the tension in the string, and this tension is what provides the centripetal force that causes the ball to move in a circle.
