Uniform motion is motion at a constant speed in a straight line. Uniform motion can be described by a few simple equations. The distance s covered by a body moving with velocity v during a time t is given by s = vt. If the velocity is changing, either in direction or magnitude, it is called accelerated motion (see acceleration). Uniformly accelerated motion is motion during which the acceleration remains constant. The average velocity during this time is one half the sum of the initial and final velocities. If a is the acceleration, v o the original velocity, and v f the final velocity, then the final velocity is given by v f = v o + at. The distance covered during this time is s = v o t + 1/2 at 2. In the simplest circular motion the speed is constant but the direction of motion is changing continuously. The acceleration causing this change, known as centripetal acceleration because it is always directed toward the center of the circular path, is given by a = v 2/ r, where v is the speed and r is the radius of the circle.