|MadSci Network: Physics|
Rockets work due to something called Newton's Third Law of Motion. This is commonly stated as "For every action, there is an equal and opposite reaction". In layman's terms, if I push something forward, it must exert a push "back" onto me. In the case of a rocket, we use a fuel (like liquid oxygen + liquid hydrogen, for the Shuttle). The fuel is allowed to react chemically, producing a product (water) and releasing energy. The energy goes into heating the product to a high temperature. Since temperate corresponds to average dispersion in kinetic energy, the molecules of the product start to dash around at a tremendous speed in essentially random directions. Now, some of these molecules happen to be heading toward the exhaust nozzle anyway. They simply escape, and don't concern us. Some others happen to moving directly away from the nozzle. These molecules smash into the front wall of the fuel tank and rebound with [in essence] no loss of energy. But now they're pointing at the nozzle, and they too can escape. But lo! They've changed direction. The wall must have exerted some force on them to do this. By Newton's III, that means they in turn pushed on the wall. Since the wall pushed them rearward (toward the nozzle), they pushed the wall forward ... and the rocket accelerates in that direction. What about the molecules who happened to be moving side-to-side (instead of toward the nozzle or away from it)? Well, they bounce into a wall, rebounded, bounce into the opposite wall, rebound, etc. But since there are a lot of these collisions, and since they point every which way, they tend to cancel out. The rocket doesn't feel any net side-to-side pushes. (Our earlier molecules had an asymmetry ... they could escape and never rebound again.) Of course, since the fuel eventually escapes, you have to keep supplying more if you want the rocket to accelerate. If you just want it to coast .... well, that's a story for another time.
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