Why, according to Newton's laws, can superman not fly?
He needs to expel gases or flap his cloak
This is due to Newton's third law which is typically recited as "for every action, there is an equal and opposite reaction". Some more meaning can be gained from the following rules:
- forces always come in pairs
- each force acts on a different object
- forces act in opposite directions
For a jet plane, this is achieved by expelling hot gases from its jet turbines. The hot gas are forced out from the engine at high speed. The engine exerts a force on the gas molecules, which in turn exert a force on the plane in the opposite direction (hence thrust). Similarly, birds wings exert a force on the air beneath them, which in turn exerts an upwards force on the bird.
So this would suggest superman needs to either:
- expel gases which I haven't seen (or hear) him do in films; or
- flap his cloak quickly (again not seen in films)
The other alternative is using the magnetic field of the earth (if he was magnetised he could align himself against the earth's field and be repulsed , but this would not be strong enough to move him)
I expect though he has an anti- gravity mechanism in his boots which solves the problem. Don't let science ruin a bit of escapism!