An object following a circular path can be covering the same distance along the circle's circumference with every passing minute, giving it a constant speed. To maintain a circular path, though, its direction must be constantly changing. Since a change in either speed OR direction means a change in velocity, the object's velocity is not constant.
Why is it useful to define velocity that way? Because Newton's laws of motion relate any change in VELOCITY (not just speed) to the application of a net force. Even though circular motion can be at a constant speed, its changing velocity requires that a (centripetal) force be present to deviate from a straight line and maintain its circular path.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor
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