normal reaction force 是不是「一定」能抵消物品加在平面上的力，使到物品不會離開平面？)

Question

即是問：在一塊傾斜度是Θ的平面上，
normal reaction force 是不是一定= mg sin Θ？

還有，如果有另外一些力加在物品上，normal reaction force會不是mg sin Θ嗎？
(即是問回標題的問題：normal reaction force 是不是「一定」能抵消物品加在平面上的力，使到物品不會離開平面？)

Answer 1

There is something wrong in your question. The [normal reaction] force and the [force acting on a plane] are froces that act on different bodies. The former acts on the object itself and the latter acts on the plane. They are a pair of action and reaction forces, but act on different bodies. There is no question of whether they [cancel or not].

Also, on an inclined plane, the normal reaction acting on an object placed on it is mg.cos(Θ).