A particle has no extent so that its position is a point. 質點沒有廣度因此它的位置是一個點。
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. 當一質點以勻角速度作圓周運動時,則它在任意直徑上的投影的運動便是簡諧運動的一例。