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Why Is It That An Apple Will Fall To The Earth?

  1. Jul 6, 2016 #1
    Why Is It That An Apple Will Fall On The Earth And Not That The Earth Will Live It Position And Move To Strike The Apple.is it just because that Earth has gravity. and Earth is a huge place very big material?
  2. jcsd
  3. Jul 6, 2016 #2
    Both the apple and the Earth have gravity. The Earth is much more massive, so has a larger gravitational 'pull'.

    In fact, due to Newton's third law, the force acting on the apple and the Earth is the same. However, since the mass of the apple is considerably less than that of the Earth, we don't notice the Earth's motion (it is so small, we can ignore it).
  4. Jul 6, 2016 #3


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    Actually they both move, just the Earth move by a negligible distance (ratio of their displacements being a function of the ratio of their masses).
  5. Jul 6, 2016 #4


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    We tend to talk about the apple moving because "the Earth doesn't move" is a convenient convention for every day use. As Borek notes, in Newtonian gravity both Earth and apple accelerate towards each other, but the acceleration is proportional to the mass of the other body - and the Earth is around 1025 times more massive that the apple. In General Relativity both objects are in free fall so there is no reason to prefer one as moving or the other.

    Typically we'd talk about the apple moving because that's the human way to think about it. You can use any other perspective you like. For example, we usually say that the moon is orbiting the Earth. In fact, both orbit around a point called the barycentre, exactly because formally we think about the Earth and moon going around each other, not one staying still and the other going round it.
  6. Jul 6, 2016 #5
    In a geocentric reference frame, the apple moves and the Earth doesn't. In the Malus domestica-centric frame, the Earth moves and the apple doesn't.
  7. Jul 6, 2016 #6


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    Yep, and from a frame of reference in which the moon is at rest, they both move.
  8. Jul 7, 2016 #7


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    Lol... it wasn't stated as a fact, but I presume the apple was hanging, or more correctly, supported by a tree... the tree would have a particular mass before the apple fell, but would lose "one apple mass" when the apple separated... wouldn't the tree play some role in this dynamics ?

    The tree did become less massive...
    Last edited: Jul 7, 2016
  9. Jul 7, 2016 #8


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    Staff: Mentor

    The tree is part of the Earth in this scenario.
  10. Jul 8, 2016 #9
    thank you now its clear to me :smile:
  11. Jul 9, 2016 #10
    The geocentric reference frame is not an inertial frame* so the Earth is moving in absolute terms, albeit with an immeasurably small amount of acceleration.

    * An inertial frame of reference is one that's not accelerating.
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