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Orbit concepts

  1. Feb 21, 2008 #1
    An object orbits a star, both items comprise the system. Im looking for confirmation of a couple of concepts regarding orbits.

    As the object slows down, does the kinetic energy of the system decrease, and does the total energy of the system decrease?
    As the kinetic energy of the system increases, does the gravitational potential energy decrease?
  2. jcsd
  3. Feb 21, 2008 #2


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    Welcome to PF, Astros.
    I am no expert in any of this stuff by anyone's stretch of imagination, but I'll throw in my 2 cents anyhow.
    To start with, no situation in the universe constitutes a true closed system... except for the universe itself. There are always gravitational, electromagnetic and possibly (:uhh:) other effects from other areas. Even the perpetual popping in and out of virtual particles indicates interference. That having been said, those effects are pretty much negligible in the scenario that you propose.
    If it was truly closed, then the system could suffer no loss of total energy. What is lost in one area is gained in another.
    As for your last question, either it's too vague or I just don't understand it. To my thinking, more information about how the kinetic energy is increased would be needed.
    Hang tight... someone who knows what he's talking about should be along shortly.
  4. Feb 21, 2008 #3
    "someone who knows what he's talking about" ... or just me.:wink:

    astros10 - why are you suggesting that the object slows down? Unless there's some mechanism for reducing its energy, then it will just continue in an unchanged orbit, like the Moon around the Earth, or the Earth around the Sun. You'll have to be a little more specific about the scenario you have in mind in order to get a good answer.
  5. Feb 21, 2008 #4
    Perhaps my questions were phrased poorly. If it is an elliptical orbit, which I should have specified, then the object would slow down as it moves away from the star or speed up as it approaches a point closer to the star, atleast that was my understanding. If it was slowing down, is it true that the kinetic energy of this system is decreasing, and if so would that mean that the gravitiational potential energy would be increasing and vice versa for speeding up? And would the total energy, kinetic + potential decrease at any point of the orbit, or would it stay the same?
  6. Feb 21, 2008 #5
    Ah, I see what you mean... Well I would be interested in hearing the answer aswell although I am interested if the gravitational potentional energy increases/deacreases by simply placing the planet at different distances from the star, since the gravitational field strength differs depending at which distance you are from the source, the energy should differ too.

    Last edited: Feb 21, 2008
  7. Feb 21, 2008 #6
    Yup, you've got it. If there's no way for energy to leak in or out of the system, say by "borrowing" energy from some third body, then the object's total energy is conserved, so the decrease in kinetic energy exactly equals the increase in gravitational potential energy (and the reverse as the object goes from apoapsis to periapsis, which are the generalizations of apogee and perigee).

    In reality, there are always other effects that do inject or remove energy from the system, but this is good for a first approximation.
  8. Feb 21, 2008 #7
    Oh -- yes, the grav. pot. energy is just a function of the distance from the center of the grav. field, since it represents the amount of work required to move the object from distance r1 to distance r2.
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