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Universal gravitation

  1. Jun 3, 2005 #1
    Hello I'm a beginner in physics and came across an explaination for Gravitation, it explains it using the moons orbit around earth as an example, they also say firing a projectile at a certain velocity can put that projectile in orbit, but they confused me with that one, can anyone help? this is the link (its only one page) http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

    What I don't understand is how the moon doesn't get accelerated down to the earth's surface by the earth's gravity, I know that the earth's gravity keeps it in orbit (obviously) but don't understand why. The only reasons I can think of is that for instance you hit a ball in the vaccum of space with no other forces acting on it and that ball will travel at that velocity forever. Is that whats happening to the moon in space? what keeps it orbiting?

    I know about the universal gravitational constant and that tells me there is attraction between the earth and moon but perhaps not enough to send the moon crashing to the earth and not too little to let the moon escape orbit, is that possible?

    Its seems like a relationship between velocity and gravity that would explain it.
  2. jcsd
  3. Jun 3, 2005 #2

    Chi Meson

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    Circular orbital motion is a combination of two phenomena. First there is the law of inertia, which you seem to grasp Ok already: an object, once moving, wants to maintain that straight line motion. Think of the moon as "wanting" (for lack of a better word) to go in a straight line.

    But the gravitational field of the earth exerts a force on the moon. The direction of the force is perpendicular to the direction the moon is moving at that moment. Since this force is perpendicular to the motion, it will not speed up or slow down the motion of the moon, it will only change the direction of motion.

    The end result is that the moon goes is a circular orbit (well close to circular, the actual orbit is elliptical, and this makes things a little more complicated, but the general idea is the same). The force from the earth is only enough to make the moon continually change its direction of motion, but it never speeds it up, slows it down, or pulls it any closer tot he earth.
  4. Jun 3, 2005 #3
    ah, thank you that is a much better explaination. Heh I felt a bit of a dummy asking such a question but you got to start somewhere!.
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