In a comic book, superman carries a bus (with a bomb on board) up into the air, hovers for a second and then trows the bus over the horizon while he stay hoverin . how does this situation break the law of conservation of motion? what should have happened instead? why?

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.