A point on the rim of a unit circle rolling without slipping traces a cycloid. The center translates by the same distance as the rim unwinds, combining uniform translation with circular motion.单位圆无滑动滚动时，圆周上一点描出摆线。圆心的平移距离等于圆周展开的弧长，因而轨迹由匀速平移与圆周运动叠加而成。
The gold radius tracks the marked rim point. At each ground contact, translation and rotation cancel its instantaneous velocity, forming a cusp.金色半径跟随圆周上的标记点。每次接触地面时，平移与旋转带来的瞬时速度相消，轨迹形成尖点。