(2) Rotation: All particles describe circular paths around a line called the axis (ti) which may fixed or may changing its position during the motion. (转动:刚体上各点都绕某一轴 作圆周运动,转轴可能是固定 的也可能在运动中改变位置。) ∵ 圆周运动All particles describe circular paths around a line called the axis（轴） which may fixed or may changing its position during the motion. (2) Rotation: (转动：刚体上各点都绕某一轴 作圆周运动，转轴可能是固定 的也可能在运动中改变位置。) 圆周运动