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If the sun's gravity is strong enough to pull a comet from the kuiper belt why does it not pull the planets into it?

I was told that the sun's gravitational pull is so strong that it pulls comets in from the kuiper belt, but why does it not do the same to the planets? Since a comet is so small in size ,mass and density and so far out how is this possible? An old teacher of mine said it has to do with the planets speed and its own gravity, but doesn't a comet move at a faster speed than the planets? Does the fact that comets do not have a gravitational field play into this? Or am I being mislead? This may be a basic question but I have not found an answer that explains this. This question and a few others is making me think I need to take astrophysics in college . Thank you.


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    Nov 15 2012: The key to any orbit is velocity: it you can move something fast enough in the correct direction, you can get an orbit (think Tether-ball with a really elastic rope). Gravity accelerates the earth toward the sun at about 6mm/s/s, but the earth is moving about 30 km/s in a different direction (almost 90 degrees from that acceleration) and is 1.47E11 meters away. With just that velocity, after a second passes the earth would move 30km at that angle, which means its distance from the sun is now... 1.47E11m PLUS 3mm. (remember that for the sides of a right triangle, [ A^2 + B^2 = C^2 ])

    Now for the acceleration. You know [ d = 0.5 * a * t^2 ], so the acceleration for that second moved us... 3mm closer to the sun. We are once again 1.47E11m away. There are other things going on, but keep in mind that calculus was invented specifically to solve this problem!
    • Nov 15 2012: Thank you, Thought it was something like that, but dwell too long on one issue and you start to over-think things and then start pondering other things and trying to make it fit into the original question.
      Like I said before my science teacher doesn't know much about space and what makes it tick, He does know a great deal about geology.
      What erks me is that I believe if your going to become a H.S. science teacher you at least be well versed in what you plan on teaching.
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        Nov 15 2012: I know what you mean. If you ask me, the phrase "that's just the way it is" defeats the whole purpose of teaching something! It might seem counter-intuitive, but ask you mathematics teacher if your science teacher can't explain something. Very often, it's because the situation has something special about it like this.
        • Nov 16 2012: Thanks for helping my son figure this out, I on the other hand am not a smart man when it comes to these things. The way his teacher brushed him off was very disappointing. But it sheds a bright light onto why America has fallen behind in the science department. When the teachers don't care its the students and the future that end up suffering.
          Again I appreciate you helping him, he was lit up when I came home. Proud to have gotten the answer to his question and even more proud to understand it too.
          makes me very happy and proud to see him grow up to want to learn about this stuff and to want to excel at it. He is smarter than I and that's exactly why i push him to study so hard, to not struggle through life as I have. - Dan

          P.s. expect more questions from him, as he would like to get a job in one of the astro science fields

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