Is it possible for even a very powerful government to control the mine of its citizens so thoughtly that they no longer believe in the law of gravity or others scientific facts?

I can't write your essay for you but I can give you a general answer. Orwell's dystopia is one of strict control and intense manipulation. The Party seeks to re-write the past to control the future. O'Brien's argument about believing anything the party says is rooted in the mind: if your mind believes it, then it is true. I'm not sure how a brainwashed individual interprets the floating of a body but the mind would do everything in its power to validate what it wants to believe. In the end, Winston's mind was turned into scrambled eggs through torture and he loved Big Brother: I suppose anything is possible.