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Gravity and Atraction.

  1. Oct 4, 2009 #1
    I have a question, concerning gravity and orbits.

    We are now orbiting around the sun, so does the moon around us.

    Why isn't the moon atracted, like us, to the earth without orbiting around it?
    What do objetcs orbit instead of being atracted directly?
  2. jcsd
  3. Oct 4, 2009 #2


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    Staff: Mentor

    The moon is pulled toward earth in the same way we are. The only real difference is that the moon is also moving tangentially, and it just so happens to be moving at the right speed tangentially so that when falling toward earth, it never hits it! That's what an orbit is.
  4. Oct 4, 2009 #3


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    If you pointed a cannon at the horizon (i.e. horizontal) and shot the cannonball hard enough, it too would orbit the Earth just like the Moon. (assuming you shot it from a high enough altitude to eliminate air friction.)
  5. Oct 4, 2009 #4
    Here's another way to think of it: If the earth were flat with the same gravitational attraction then the moon would indeed hit the earth. However, because the earth is a sphere and the moon is also moving tangentially, the surface of the earth curves away at the same rate the moon is falling.
  6. Oct 4, 2009 #5
    That's nice idea, and you can see that the closer to the Earth, the faster the satellite would 'fall' because the surface of the Earth curves faster when you are closer to it. In reality, the closer the satellite, the faster it orbits (higher angular velocity)
  7. Oct 5, 2009 #6
    I like to think of it just as equilibrium of gravitational and centrifugal force. Down to earth example, spinning the stone on the end of a rope. Rope tension being the gravity.
    Things would make much more sense if there were rope tied to the Moon! I hate gravity. :)
  8. Oct 5, 2009 #7

    D H

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    That is a common mis-explanation. For one thing, there is no centrifugal force in an inertial frame. The centrifugal force explanation only comes into play when one is look at things from the perspective of a rotating frame in which the Moon is stationary. For another, this explanation does not work for elliptical orbits. Orbits are never perfectly circular.
  9. Oct 5, 2009 #8
    Well yes, I was in fact just offering a differnet point of view, with fictional centrifugal force. True about elliptical orbits also. There is no eqilibrium there and kinetic energy does change. Never really tought about it that way, thanks.
  10. Oct 5, 2009 #9
    Thank you guys, you really helped me on that one!
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