|MadSci Network: Physics|
All such objects have mass. According to Newton's Theory of Gravity, the earth exerts force on such objects. The force is proportional to the object's mass, inversely proportional to the distance from the center of the earth to the object and directed toward the center of the earth. Thus, this force is never zero except in the limit of being infinitly far from the earth. If an object is moving away from the earth, and the force of the earth on the object is the only force acting on the object, then, according to Newton's second law of forces, it will accelerate toward the earth. This will reduce the speed of the object; and unless the original speed is equal to or greater than escape velocity, the object will eventually stop and begin moving toward the earth with increasing speed. If an object is seen moving away from the earth at a constant speed, then we must conclude that the net force on the object is zero and therefore there are one or more forces acting to counter the force of the earth. Toss a rock in the air. Notice that the faster it is moving, the higher that it goes. When the initial speed reaches escape velocity, then the rock goes away, always slowing down, but never stopping. [Moderator note: You may want to look at these articles in our archives which discuss escape velocity:
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