This breaks Newton's Second Law: For every action there is an equal and opposite reaction. The force exerted on the bus by superman should also be exerted on superman by the bus. Using the equation F=m*a, since superman's mass is so much less than the mass of the bus, he should experience a much higher acceleration than the bus does. So if the bus goes over the horizon, then superman should go MUCH further the other way. The caveat is that superman isn't able to change his density at will, which I believe is true in some storylines.
