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Why is the Earth a perpetual motion machine

  1. Aug 8, 2008 #1
    Seems like an odd question - but if your an astraunaut in earth orbit and travel with it around the sun - your going different speeds around the sun but always traveling the same distance away from any two points on the sun every second (keplers #2)

    the how = closer you are to sun, the faster you go around it, farther you are, slower you go
    the why = (i dont know - it has to do with distance messing up the influence, but that's not why it it does it)

    Now whats obvious to me is theres some sort of influence there that's invisible and sometime ago a man made up and invented a random name to call it and called it gravity - but that doesn't answer why the Earth is so obsessed with always traveling the same amount of space every hour relative to its distance away from the sun (keplers 2nd law).
    For example, on Earth, when you put two items in a bowl and whirl em around.. they tend to keep coming closer and closer together .. in space they would do this because of attraction.. but I've never seen nor heard of any instance where the Earth keeps coming closer and closer to the Sun in its annual ellipse. It just keeps going like a perpetual motion machine For some reason Galaxies merge and other items tend to collide.. and meteoroids keep coming closer and closer to the Sun on each pass by.. but the Earth keeps at it's same ellipse. Why is that?

    (I'm wondering if it has to do with the Sun just being a foci and not the center of the earth's path, but why do other things keep coming closer then?)
    Last edited: Aug 8, 2008
  2. jcsd
  3. Aug 9, 2008 #2


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    Hi eratosthenes2! :smile:

    The Earth keeps going because its tangential velocity balances its centripetal gravitational acceleration towards the sun.

    When galaxies collide, it is because they do not have sufficient tangential velocity to balance the gravitational acceleration! :smile:
    The two items in a bowl come together because of various resonances, not because of any attraction between them.

    In fact, though, in the early solar system, the planets did sometimes attract each other (that's how we got the Moon! :wink:), and indeed that's how the planets formed in the first place.

    The orbits of the eight (nine?) planets we have today are what is left after all the resonances settled down. :smile:
  4. Aug 9, 2008 #3
    I asked a question similar to this a while ago. I'm thinking that what you're wondering is: Why are systems, such as solar systems and galaxies, the way they are? Why are they organized the way they are rather than entering a state of chaos and disorder? I still do not have much of an answer to that.

    The Earth is not exactly 100% a perpetual motion machine...the reason why its relatively stable is because its at a state of energy in equilibrium. This does not necessarily imply that all systems are stable. Apparently, the universe is expanding...so at the moment we cannot imply its stability just yet.
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