重力是一种最常见的力，也是和我们关系最密切的力之一．与其它力的一个不同之处，是它不仅具有大小、方向，而且其方向是特定的．初中物理第一册（九义人教版）上对重力的方向是这样叙述的：“用一根线把物体悬挂起来，物体静止时，线的方向跟重力方向一致，这个方向叫做竖直 Gravity is one of the most common forces, and it is one of the most closely related forces. One difference with other forces is that it not only has size, direction, but also direction. The direction of gravity in the first volume of the junior high school physics book (Jiyi Renjiao) is described as follows: “A line is used to hang the object. When the object is stationary, the direction of the line is the same as the direction of gravity. This direction is called vertical.