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Orbit or fall

  1. Jan 15, 2007 #1
    objects near the earths surface fall towards the center of the planet due to the force of gravity caused by its mass. once you get far enough away from the planet, the object will orbit. what is the force that is causing the orbit, keeping the object from falling to the surface? the force of gravity is definately less as the object moved farther away, but it is the only force acting so why the circular motion ?
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  3. Jan 15, 2007 #2
    What you call "falling" is actually orbiting (or perhaps the other way 'round). It's just that the surface of the planet sometimes gets in the way.
  4. Jan 15, 2007 #3


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    Why, what other forces are required for circular motion besides of course, the centripetal force?
  5. Jan 15, 2007 #4
    Go forward, but have something pulling you always to one side or the other.
    That makes a circular path.

    With space shuttles and satellites, the initial "go forward" force is provided by rockets, which never go straight up, but always eastward. Add to that whatever tangential velocity you had just sitting on the ground (which itself is turning). The Earth's gravity does the pulling to one side.

    If your tangential (to the surface of the Earth) velocity is too slow, you spiral down and burn up in the atmosphere or crash. If your tangential velocity is too fast, you either go into a higher orbit, or into an elliptical orbit (I don't recall exactly how it works in those cases). if it's way too fast, you go off into space, possibly to be captured by another big object (after a zillion years or so).

    Height above ground alone will not put you into an orbit.

    I understand the notion of the "slingshot effect" whereby they can change the direction and velocity of a deep-space probe by aiming it to go near (but not at) a large planet, but I'm fuzzy on the elliptical orbit business, and how critical it has to be to keep from spiraling down on one hand, and escaping into interplanetary space on the other. I know comets do it all the time, and they're really dumb.
  6. Jan 15, 2007 #5


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    Are you moving in circular motion right now?
  7. Jan 15, 2007 #6


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    "Orbiting" the earth is essentially falling toward the earth but missing! Your first statement, that "once you get far enough away from the planet, the object will orbit." is not true. If you place an object far from the earth but motionless relative to the earth, it will fall down (eventually hitting the point on the earth that has rotated under it. IF your object has sufficient "horizontal" motion, it will "miss" the earth and, instead of hitting a point on the earth, rotate around it. That is true whether you are far from the earth or near it. Of course, the farther you are from the earth, the more time you have to "miss" so you don't need as great a horizontal component of velocity. Also, farther away from the earth air friction becomes negligible.
  8. Jan 15, 2007 #7


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    Yeah, my thinking there wasn't quite right (good thing I was vague...). If you fire a projectile vertically, it won't come back to where it started because of the coriolis effect, so there is no amount of energy you can put into it vertically to put it into orbit.

    If you were to hoist an object up to geostationary orbit via a space elevator, the elevator provides a horizontal force to accelerate the object (which, btw, adds some more complexity to the structural issues of building one).
  9. Jan 15, 2007 #8
    it doesnt orbit the earth just by being high up, it must be going forward at a certain speed, and orbit is acheived when it goes forward with enough velocity that instead of falling straight down, it falls in a curved line, the faster the acceleration, the 'less curved' the path appears to be, until you are going forward with along the curve of the earth. if you have enough acceleration, you will actually curve away from the earth, and slowly drift away with each orbit, much like the moon is doing with the earth. every year its an inch farther
    Last edited: Jan 15, 2007
  10. Jan 15, 2007 #9
    Last edited by a moderator: Apr 22, 2017
  11. Jan 15, 2007 #10


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    Well, would it need acceleration forward? when it gets in orbit acceleration would only push out out of orbit. A satelite has no acceleration.
  12. Jan 15, 2007 #11


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    It is always accelerating towards the center of the earth. A tangential acceleration will cause the body to change orbit. That means the body will "fall" closer to, or further from the earth.
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