现实世界中存在着许多具有相反意义的量 ,例如 :物体做直线运动时 ,从一个定点出发前进或后退3公里 ,虽然路程相同 ,但是最后到达的位置却不同。这个事实可以用下图来表示。设物体原来的位置是 O,前进 3公里到达 A,后退 3公里到达 B。显然 ,A、B这两点虽然离开O点的距离相同 , In the real world, there are many quantities that have opposite meanings. For example, when an object moves in a straight line, it moves forward or backward 3 kilometers from a fixed point. Although the distance is the same, the last arrived position is different. This fact can be expressed in the following figure. Let the original position of the object be O, advance 3 kilometers to reach A, and retreat 3 kilometers to B. Obviously, the two points A and B are the same distance apart from point O,