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Exactly Why doesn't the moon fall and strike the earth?

  1. Nov 13, 2006 #1
    Is it because the moon is actually falling in free fall TOWARDS the earth but it is moving in a circular path the same as the earths path??
     
  2. jcsd
  3. Nov 13, 2006 #2

    Office_Shredder

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    Essentially, the moon is moving so fast, that it travels past the curve of the earth before it would strike. Imagine if you threw a baseball at 20,000 mph.... so it would fly off, and as it fell, the earth would curve, so it would never actually strike the ground.

    You could ask the same question of the planets orbitting around the sun, by the way
     
  4. Nov 13, 2006 #3

    BobG

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    All moving objects in the vicinity of Earth travel in an elliptical orbit around the Earth's center of mass unless atmospheric drag or some other force sucks energy out of the orbit. How close the objects come to the Earth's center of mass is the key. If the closest point in its orbit happens to be below the Earth's surface, it hits the ground.

    The Moon is travelling fast enough that it's closest point is well outside the Earth's atmosphere. The more energy (kinetic and potential energy combined) an object has, the larger the orbit, and the further the object stays from Earth. So it's a balance between speed and position - a object closer to the Earth has to travel faster to keep it's closest point outside the atmosphere than an object that starts out far away.

    While virtually all orbits are elliptical (the odds of an object's orbit being perfectly circular and staying perfectly circular are virtually nil), the Moon's orbit is almost circular.
     
  5. Nov 13, 2006 #4

    DaveC426913

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    I think this is missing the OP's point.

    I think an answer that would benefit him is why things orbit at all.


    An orbit is merely a very specific set of circumstances that can befall two objects that interact.

    All objects have some sort of initial velocity wrt other objects. There are three general cases:

    1] The relative velocity is large, while the gravitational attraction is small.
    The two objects, as they near each other at high speeds, will influence each other gravitationally - they'll deflect their straight paths towards each other. But their v is so high that they pass each other and continue away from each other. (This is a one-time occurence, the two bodies pass and never see each other again. Not a lot of real-world examples for that reason.)

    2] The relative velocity is small while the gravitational attraction is large.
    The gravity of the two objects overcomes their velocity and they collide. (Any impact is a good example such as Meteorites)

    3] The relative velocity is nicely balanced with the gravitational attraction.
    The objects come near each other, swing around, and neither collide nor escape. They do a dance aorund each other for a short or even very long time. (Examples include all planets orbiting stars, all moons orbiting planets, etc.)


    It is important to note that these two forces - gravitational attraction and relative velocity - are not necessarily unrelated. For example, the planets and the Sun formed out of the same rotating disc of dust and gas. Anything that didn't succumb to 1] or 2] ended up as 3]. So it's no coincidence that the planets orbit the sun just right.

    This is also why - despite 3] being the most finely-balanced of the three cases (which would suggest it's the rarest) - it is, in fact, quite a common occurence in our universe.
     
    Last edited: Nov 13, 2006
  6. Nov 13, 2006 #5

    Office_Shredder

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    On the other hand, if things have struck or escaped from each other, they're no longer around for us to witness, so it makes sense we would see orbitting (I feel like I'm mispelling that...) more than anything else
     
  7. Nov 13, 2006 #6

    DaveC426913

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    I made this point in 1]. But there's no dirth of examples of 2] - despite them being one-time events.
     
    Last edited: Nov 13, 2006
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