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 - Converting from 6 Volt to 12 Volt - by Chris Pratt. It's a common problem for old machines especially when the weather starts to dip below 40 and the tractor isn't fired up as often. You are probably used to the 12 volt Diehard, charged with 200 amp alternator of your 3/4 ton that always is ready and always starts. It's this type of experience that brings about the many queries we get for 12-volt conversion kits. Since only a few kits are available (we sell kits for the
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