如果an object 站在地上, R(normal force) balance 左 W (weight) 所以the object remains at rest.
但係 w 的reaction force 係個oject 拉住地球果個力, 我可唔可以話係 呢個力balance 左 weight 呢個力, 所以the object 沒有向下加速。
而 R 的reaction force 是object acts on the ground 果個力
我又可唔可話這個力 balanced 左R 呢個力, 所以the object 沒有向上速
THx