Let me give it a shot. The formula describing the relationship between force and acceleration is "F=ma", where F is the force applied to an object, m is the mass of the object, and a is the rate of acceleration. So to accelerate an object, be it a car, airplane, baseball or rocket ship, you must apply a force to it. Now on earth, objects are subject to a lot of forces (gravitation, wind, friction, bat swings, etc.) but in space there are essentially no forces on an object, unless it creates them itself.
OK, we want to accelerate our rocket ship, so we need to apply a force to it. But how can we apply a force when there's nothing to "push" against? Well, if we accelerate a DIFFERENT object, we have to create force to do it, and that same force will be applied in the opposite direction to our spacecraft. The "different object", in this case, is the hot gas ejected from our rocket motor. We create force by accelerating that gas. Now we COULD accelerate a heavy object (say a cannonball) to a low velocity to create our force (remember F=ma), but heavy things are expensive to get into space. So it makes since to accelerate a light object to a very high speed. Each molecule of gas accelerated to high speed results in a small force applied to the spacecraft. Accelerate enough gas molecules and you can create a significant force.
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Today's Featured Article - Fasteners: The Nuts and Bolts of Nuts and Bolts - by Curtis Von Fange. The nuts and bolts of nuts and bolts is an interesting and essential piece of knowledge that applies to our older tractors. An improperly torqued capscrew on an engine head or a shear bolt that is too hard on the driving shaft of a bushog can create havoc and make an expensive and uncalled for repair. Let�s examine the purpose and design of these fasteners in order to ensure their proper use. Fasteners are probably one of the aspects of mechanics that is given the least amount of thought.
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