About satellites, velocity, gravitation

  • Thread starter ay2k
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  • #1
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Main Question or Discussion Point

A satellite is in a particular orbit. Too put it in a higher orbit(greater distance from earth), we increase its velocity. I suppose this statement is correct.

But when we fire an object from earth, its velocity is lesser for greater distances from earth.


Can anyone please simplify this apparent contradiction.

Any link would be highly appreciated.
 

Answers and Replies

  • #2
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excuse me...
i need an urgent reply...
 
  • #3
tiny-tim
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Hi ay2k! :smile:

But when we increase a satellite's velocity, its velocity will be less as it gets further from earth.
 
  • #4
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please elaborate...
 
  • #5
tiny-tim
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please elaborate...
It sweeps out equal areas in equal times (one of Kepler's laws), so when it's nearer the planet, it goes faster, and when it's further, it goes slower. :smile:
 
  • #6
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But when we fire an object from earth, its velocity is lesser for greater distances from earth.
This doesn't necessarily have a direct correlation to an orbiting satellite. Nor would this be true in general. If you were to throw a baseball straight up, then yes the velocity will decrease as the baseball gets further away from you.

Now throw the baseball straight ahead. It will eventually hit the ground. Throw it a bit harder now. It will take longer for it to hit the ground. Now throw it REALLY hard. If you're good it might just hit you in the back of the head.

Now try the same experiment on a really high mountain. Now you won't have to throw as hard. Thus we have confirmed tiny-tim's assertion...
 
  • #7
rcgldr
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The velocity is increased while the satellite is in a lower orbital path, but the end result is the satellite ends up in a higher orbital path, but at a lower speed, once it stabilizes into it's new orbital path. Assuming the path isn't intended to be elliptical, it takes two bursts of thrust; the inital burst to get it into a higher on average orbit, and a second burst to get the orbit to be circular.
 
  • #8
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The whole prinicple of higher orbit - slower speed has to do with the law of conservation of angular momentum. The momentum has to stay constant (p= mvr), with r the distance between the satelite and the earth, wich means if r increases v has to decrease and visa versa.
 
  • #9
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The centripetal force (gravitational attraction of Earth in this case) also decreases as well the farther you get away from the center of the Earth.
 
  • #10
rcgldr
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The whole prinicple of higher orbit - slower speed has to do with the law of conservation of angular momentum. The momentum has to stay constant (p= mvr), with r the distance between the satelite and the earth, wich means if r increases v has to decrease and visa versa.
In order for this to be a closed system, and for angular momentum to be conserved, the velocity and mass of the spent fuel expelled from the rocket engine that propels the satellite into orbit needs to be included. If the expelled fuel is ignored, and only the satellites movement considered, then angular momentum will not appear to be preserved.
 

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