Because velocity is the rate of motion of an object in a specific direction, either an increase or decrease in the object's speed or a change in its direction of travel causes its velocity to change. This change in velocity is defined as an acceleration, which according to Newton's laws of motion occurs because of an unbalanced force acting on the object.
Speed, in contrast to velocity, only states how fast an object is travelling, and thus the speed of an object can remain constant even as its velocity fluctuates. In circular motion, where the object moves in a perfect circular path, the speed of the object remains constant, but its direction is constantly changing because of acceleration toward the center of the rotation; thus, its velocity is changing. Newton's second law states that acceleration is caused by an unbalanced force, is proportional to the force and its in the same direction as the force. Acceleration in circular motion is caused by a centripetal force acting in the direction of the center of rotation perpendicular to an object's direction of travel. In contrast, during a free fall, the force of gravity accelerates the object, causing its speed to increase, while its direction of motion is constant. In this case, the velocity is still changing.