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Name: Eve
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Question:
Why does the speed of the orbit slow down the further away you go from the source of the gravity?



Replies:
Eve,

For a constant radius, there is only one speed that an object can travel to maintain orbit. If it goes any faster or slower than this speed it will either go off into space (faster) or crash down to Earth (slower). Your question is about why the speed has to slow down when the orbital radius (how far way the satellite is from Earth) increases. The velocity of the satellite to stay in orbit is determined by
1. gravity (the universal gravitational constant, actually),
2. the mass of the object that the satellite is traveling around (in most cases, Earth) and
3. the distance between the center of the orbit and the satellite (orbital radius) (which would be the distance from the center of the Earth to the satellite in most cases). The velocity is, specifically, inversely proportionally to the square root of the radius. What does this mean? In practical terms, it represents that as you get further away from the source of gravity (i.e. the center of Earth), that the force of gravity weakens. You know that this is true because in space you float around with little to no gravitational force to pull you back to Earth even though you are still close enough to see the Earth. In fact, every time you double the radius, you must slow your speed down by a factor of four in order to stay in orbit. If Earth's gravity is pulling on you less and less, then you must slow down so that you do not fly out of orbit and off into space.

Matt Voss



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