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A gravity problem i came across.

  1. Dec 13, 2012 #1

    Im sure we're all familiar with the fact that energy can't be created, only changed into diffrent states. However, i've been thinking about this scenario i find rather strange:
    If a space-shuttle is put in orbit around a planet or object with large gravity, the shuttle will accelerate around it. When the speed is high enough the gravity-field will not be able to keep the shuttle in orbit, and it will be thrown out into space again.

    My question is: Have energy been created? Since the shuttle have greater speed after leaving the orbit than when it entered the orbit.

    Im sure my problem is somehow invalid, i just want to get this one out of my head.

  2. jcsd
  3. Dec 13, 2012 #2

    Philip Wood

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    Gold Member

    You say, "when the speed is high enough..." Aren't you implying that fuel is being burnt (and energy transferred into kinetic) in order to achieve the higher speed? Maybe I just don't understand the set-up.
  4. Dec 13, 2012 #3
    I think you have the wrong idea. A body in orbit around another body is accelerating and acceleration is a change in velocity. However velocity is a vector, and it is the angle part of the velocity which is changing not the speed. The speed varies slightly because of the orbit eccentricity but the orbit doesn't change, apart from losing energy to friction etc.
  5. Dec 14, 2012 #4
    The shuttle will indeed have more energy, but YOU have to give it that energy. As Philip Wood said, probably by burning fuel. It's not just going to happen by itself spontaneously.
  6. Dec 18, 2012 #5
    Ok, thanks for the help!
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