Characteristics of Circular Motion

Circular motion -- whether it is revolution or rotation -- has two characteristics that hold no matter how the motion is produced. First, it is always an ACCELERATED motion. Even when a particle's speed stays perfectly constant, its VELOCITY does not, because velocity is a vector and its direction is constantly changing as the particle goes around the circle; a changing direction, at unchanging or changing magnitude, is still a changing velocity, and any change in velocity is by definition an acceleration. Second, circular motion is a PERIODIC motion: the particle keeps retracing the very same trajectory over and over, repeating its path (and, in uniform circular motion, its speed and position) after every complete revolution. These two properties -- always accelerated, always periodic -- are the foundation for everything that follows: the accelerated nature is what forces us to identify a real (or, in a rotating fr …