Originally posted by eatmybishop
forgive my ignorance on this one, but two questions about gravity which i hope someone can answer, if the gravitational pull from the sun keeps the nine planets in their orbit, why don't they just race toward the sun, whats pulling from the other side? also, how can a comet pass through our solar system without bring pulled in by the sun's gravity, if gravity has the power to hold a planet, why not a comet? thanks
Imagine a ball at the end of a string. If you swing the string around, the ball will "orbit" you. Why doesn't it fly away? Because the string pulls it towards you. Why doesn't it just go right for you then? Because it has momentum that keeps it moving sideways as well.
You can think of planets as objects that are always falling towards the sun, but just keep missing. If you managed to stop a planet dead in its orbit and let it go, it would indeed fall right into the sun. But as it is, planets have a momentum that keeps them from falling directly towards the sun.
Many comets are bound to the sun just like planets are. Unlike planets, though, they have very stretched-out elliptical orbits that take them far beyond the orbit of Pluto before they fall back towards the sun again. But there are objects that pass through the solar system without being caught by the sun's gravity. They can do this because they are moving quickly enough to escape the sun's gravitational pull.
Imagine the ball-and-string again: if you spin the ball around fast enough, eventually the string will break and the ball will fly off. An inexact analogy, but I think it conveys the general idea: to escape the sun's gravity, an object has to be moving away from it at a certain speed. But gravity is not like a string that suddenly snaps. It's more like an elastic cord, with the difference that the closer an object is to the sun, the more strongly it is pulled in, whereas with an elastic cord the force becomes stronger as the distance becomes larger.