An object moving at constant speed in a circle is not in equilibrium. Although the magnitude of the linear velocity is not changing, the direction of velocity is continually changing. Since a change in direction requires acceleration, an object moving in a circular path has a constant acceleration towards the center of the circular path.

Recalling Newton's second law of motion, F = ma, a force is required to cause acceleration. Therefore, to have constant acceleration towards the center of the circular path, there must be a net force acting towards the center. This force is known as centripetal force. Without this force, an object will move in a straight line. Figure 10 illustrates the centripetal force.

Figure 10 Centripetal Force