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 - A Cautionary Tale - by Ian Minshull. In the early 1950s my father bought an Allis Chalmers B and I used it for all the row crop work with the mangolds and potatoes, rolling and the haymaking on our farm. The farm and the Allis were sold and I have spent a lifetime working on farms throughout the country. I promised myself that one day I would own an Allis. That time event
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