an object in uniform circular motion has a?

Velocity, as a vector, wouldn't be constant because you're constantly changing direction.

Since your velocity is constantly changing, your acceleration must be constantly changing, and therefore is not constant.

(Keep in mind that speed and velocity are two different things. You can have a constant speed without having a constant velocity.)

If the net force was zero, then how could the object move in a circular motion? There must be a net force present to keep that object moving the way it does. (This is known as centripetal force, and it always points to the center of the circular motion.) If you think about a ball on the end of a string moving in uniform circular motion, the centripetal force would be the force of the tension in the string pulling the ball around in a circle from the center.

Constant acceleration (centripetal).