A particle has no extent so that its position is a point. 质点没有广度因此它的位置是一个点。
One instance of simple harmonic motion is that of the projection on any diametre of a point which describes a circular path with uniform angular velocity. 当一质点以匀角速度作圆周运动时，则它在任意直径上的投影的运动便是简谐运动的一例。
