[cinFLA]

Go check your physics for "On Earth there is always air or water available to push against."

[spunkets]

Re: push against

Push refers to the direction of the force. If the force is derived from an IC engine, the force out of the trans is applied to either the air, water, or some hard surface. The force is accurately described as a push. The surface it acts against also exerts a push in the opposite direction. If it's a jet the walls of the motor do the pushing, by directing the hot gases pushing on it.

[El Gato]

Just so everybody understands, it's the "push against" that you are (rightly) objecting to. The propeller or the jet don't "push against" the air, rather they move the air backwards and, by conservation of momentum, that causes the engine to move forwards. More properly it causes a force in the foward direction. The same as the recoil of a gun, which would "kick" just as much (well close enough) in a vacumn as it does down here at the bottom of the atmosphere.

[Physicist]

Go check your physics for "On Earth there is always air or water available to push against."

I'm afraid I don't understand what you're driving at, either. Could you spell it out for us?