Answer
The magnitude of the velocity is constant, but since the direction of the velocity vector changes, the velocity is not constant.
The magnitude of the acceleration is constant, but since the direction of the acceleration vector changes, the acceleration is not constant.
Work Step by Step
In uniform circular motion, the speed of an object is constant. However, since the direction of the object's motion changes continuously as it moves around the circle, the velocity is not constant.
$a_c = \frac{v^2}{r}$
Since the speed $v$ is constant, then the magnitude of the acceleration is also constant. However, the direction of the acceleration always points toward the center of the circle. Since the direction of the acceleration vector changes continuously as the object moves around in a circle, the acceleration is not constant.
