Floor is 1/2 the problem, actually. British people use floor indiscrimantly pretty much for anything flat (more or less) that people walk on.
Where as American's use floor for anything flat, that people walk on, under a roof.
American's use 'ground' for anything flat, that people walk on, that isn't covered by a roof. (part of what you learn of 'inside/outside' at around age 3)
So, depending on where you live, you would say:
He got hit so hard, he fell to the ground in Central Park.
He got hit so hard, he fell to the floor in Central Park.
Now a British person will figure out that 'ground' means 'floor'. But an american will have a question - what building in Central Park was He in?