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Zero gravityby revesz
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#1
Nov3003, 01:59 PM

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Imagine a universe with nothing in it exept for the earth. Now if you were to put a satalite up in space with a geosyncronized orbit, the gravity of earth would not pull the object down. but how can that happen if the earth is not spinning? At least not spinning relative to anything(because there is nothing), so there is no gravity at the altitude that the satalite is at. and if you were to place the satalite any further out it would just fly off into space. How can this happen?
How can the satalite have an centrifical force(or momentum), if there is nothing to be moving relative to? and why wouldn't this happen in reality, or would it? 


#2
Nov3003, 02:12 PM

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The gravity due to a body at any and every point in space is given by: [tex] F=G\frac{Mm}{r^2} [/tex] Where G is the gravitational constant M is the mass of the attracting body m is the mass of the satellite r is the distance between them 


#3
Nov3003, 02:30 PM

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If that is not true, then what prevents geosyncronized satalites from crashing to earth? If all motion is relative you may observe both the earth and the satalite as stationary, so there would be no centrifical force holding the satalite up, and so it would be pulled down be gravity.



#4
Nov3003, 04:23 PM

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Zero gravity
According to Newton's First Law, an object will go in a straight path unless acted upon by a force.
That is what would happen to a satellite if there were no gravity: it would continue in a straight line forever, and it would not be in an orbit. The satellite is geosynchrinous because it goes around the Earth once every 24 hours  the same period as the Earth's rotation. Now if you assume that the Earth isn't going through it's daily rotation, then a geosynchronous orbit would be impossible. A satellite which is now in geosynch would be orbitting once every 24 hours, and a satellite which appears to be geosynchronous (not moving tangentially) would crash into the Earth in a matter of hours. Does that make sense? 


#5
Nov3003, 07:37 PM

P: 58

Yes it does make sense, but I'm just realy confused about momentum. If its all relative then what if you view the satalite and earth while spinning really fast, the geosyncronized satalite will be orbiting around the earth and so it would have enough momentum to hold its possition or even fly off into space. I know this is not true because it doesn'y make sense, but how do you decide wether something has momentum or not?
another question: can an object such as the earth be spinning around only one axis, or can it have 2 or 3? for example take the earth, it is rotating around a line between the north and south poles, can you have rotation around opposite sides of the equator at the same time, or does this just move the axis? 


#6
Dec103, 01:43 AM

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If the Earth is spinning really fast, then the geosynch satellite isn't going to be a geosynch satellite.
The only thing special about geosynch is its distance from the Earth. Satellites in low Earth orbit (LEO) go around once every 45 minutes or so. The farther out you go, the slower the satellite goes, and the longer the orbit lasts. GPS satellites are in an orbit about 25000km up, and they go around once every 12 hours. Geosynch sats are in an orbit 36,000km up, and they go around every 24 hours. The moon is in an orbit over 100,000km up, and it goes around once every 28 days. If you take a Geosynch satellite, and speed up the rotation of the Earth to 45 minutes (somehow), then the satellite will still be orbitting once every 24 hours at an altitude of 36,000km. Nothing will change except it's title. In this case, the LEO satellites are the ones which are geosynchronous, because they go around once for every rotation of the Earth. 


#7
Dec103, 01:56 AM

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And sorry to nitpick enigma, but an orbit at LEO distance takes roughly 90 minutes and the moon is roughly 340,000 km away. You're right on the GEO distance though. 


#8
Dec103, 11:01 AM

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"Spinning" involves acceleration, not just straight line motion at a constant speed, and so is NOT relative. If the earth were the only object in the universe, people on it could still detect and measure its rotation.



#9
Dec103, 03:25 PM

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That's what I get for posting at 2AM after looking at GPS satellite data all day long (which are visible for 45 minutes) Guessed on the moon distance. 


#10
Dec203, 09:07 AM

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#11
Dec203, 01:37 PM

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The professor does that kind of stuff for a living at NRL. Based on his class there is no way I want to do that for the rest of my life... I'm blind enough now as it is. Can you imagine looking at this kind of stuff for 8 hours a day?



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