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Gravitational potential energy and Kinetic Energy

  1. Mar 24, 2007 #1
    Pe = Potential energy
    Ke = kinetic energy

    i want to clear up some confusion in me...
    firstly...it is true to say that potential energy transforms into kinetic energy as an object...correct?

    then the definition of an ORBIT is constantly falling around a planet, non-stop...

    therefore, Poetential energy is constantly being transformed into kinetic energy
    and an orbit is continuous and will never stop unless an external force acts upon it. i.e. the Pe will continue to transform into Ke for all eternity, which also implies there is infinite Potential energy...which contradicts the law of conservation of energy...

    o_O i know Pe can be calculated by the height from which it is above the center of earth, but that is not the point...

    so is there anything wrong with waht i said? or have i proved physics contradictory?
  2. jcsd
  3. Mar 24, 2007 #2

    Doc Al

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    Staff: Mentor

    It's certainly true that PE can often be transformed into KE--just drop a ball.

    I presume you mean "falling" compared to the satellite's path if the planet wasn't there--the planet and satellite exert forces on each other. The orbiting satellite is constantly changing the direction of its motion, not necessarily its distance to the planet.

    Sorry... I don't get what you're saying. What makes you think that an orbiting satellite is constantly losing PE and gaining KE? Only if it's crashing into the planet! In a circular orbit, for example, the PE and KE of the system remain the same.

    I'm afraid that is the point. If the distance between satellite and planet doesn't change, neither does their combined gravitational PE.
  4. Mar 24, 2007 #3
    Usually, the distance between a satellite and a planet do change. When the distance diminishes Pe diminishes and is transformed in Ke, conversely, when the distance increases, it is the Ke which transforms in Pe. Total energy does not change, unless others forces intervene (e.g. friction with the atmosphere).
  5. Mar 24, 2007 #4


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    Understand that neither the distance from the planet nor the speed of the satellite changes. This then implies that both the gravitational potential energy and kinetic energy do not change. Yes, it is correct to say that the satellite is constantly falling, but since it doesn't actually get closer to the planet, the gravitational potential energy doesn't change. It is not the act of falling which changes the potential energy, but the act of moving closer to the planet, which the satellite doesn't do. Hence, its gravitational potential energy of the satellite doesn't change.

    Hope this helped.
  6. Mar 24, 2007 #5
    To this I would just add one other noteworthy detail (hopefully not confusing anyone in the process). Gravitational potential energy depends on an object's distance from a massive body. In a circular orrbit this distance never changes. But in the case of an elliptical or parabolic orbit, the satellite's distance from the massive body does change, and along with it, so does the gravitational potential energy. This energy goes into the kinetic energy of the satellite. As a result, a satellite will travel faster when it is closer to the massive body that it is orbiting, and slower when it is farther away.
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