關於F=MA 和 FRICTION

The main point is that although the coefficient of static friction decreases in a wet condition, the frictional force required to pull the piece of iron forward is still smaller than the limiting friction under the wet condition. That's why your equation f=ma is still applicable.
However, if the acceleration is so large that the f (force pulling the iron piece) exceeds the limiting friction in wet condition, the iron piece will begin to slip.

我猜你所說的一件鐵東西在貨車頂是指在貨車斗吧, 而且鐵件是自由地放在車斗, 即沒有固定在車斗底部.
鐵件和車斗之間的磨擦力是 : Fr=uF
Fr = 磨擦力
u = 磨擦系數; ud = 無水時的磨擦系數; uw=有水時的磨擦系數
F = 鐵件作用於車斗的受地心吸力的作用力
= mg => m = 鐵件的質量; g = 引力加速度
從上所述, 無水時, 磨擦力
Fr d = ud * m * g
有水時, 磨擦力
Fr w = uw * m * g

由此可見, 鐵件的質量不會因天氣的變化而變化, 只有磨擦系數受兩件物件之間會否有其他物質(如潤滑油)的存在而改變. 反作用力亦不會減少.