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Name: Unknown
Status: Other
Age: N/A
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Date: Prior to 1993

If outer space is made up of nothing, then how does a space vessel get its propulsion? My guess is that the substance created by the thrusters propels the ship. I would appreciate any answers you could give me.

Your guess is correct! When we watch the Shuttle blast off, it really does look as if the exhaust gases are pushing against the Earth to force the rocket upward. But that is not what happens. The rocket would move off in the direction opposite the exhaust-gas velocity even in outer space, away from any planetary surface to "push" on. Here is why. When you blow up a balloon and let it go without tying it, the air escapes and makes the balloon fly around. Let us think about why this happens. First, suppose the balloon is blown up and tied. The air inside pushes outward on the balloon, and the balloon pushes back on the air. But the balloon does not go anywhere because the air inside pushes with the same force in all directions, so all of the forces on the different parts of the inner surface of the balloon cancel each other. Thus, there is no net force on the balloon due to the inner air pressure. Now think of the untied balloon. In this case, the net force on the balloon due to the air inside is not zero, because the force exerted on the part of the inner surface opposite the neck of the balloon is not balanced (canceled): the air at the neck rushes out instead of pushing on the balloon. Thus, the balloon accelerates in a direction opposite the neck of the balloon. The situation with a rocket is very similar. The burning fuel expands inside the fuel chamber and pushes on the inside of the chamber; since it can escape out the exhaust tubes, the force on the rocket is unbalanced and the rocket accelerates in the direction opposite to the direction in which the exhaust gas moves.

RC Winther

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