How does an apple fall to the ground
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How does an apple fall to the ground

[From: ] [author: ] [Date: 11-04-23] [Hit: ]
-No one here is answering your actual question, which is whether there is a sense in GR in which the ground rises up to hit the apple. Everyone is answering in terms of Newtonian gravity, which, of course, GR replaces.......
is it true that an apple doesn't really fall 2 the ground according to GR? but the ground comes up to meet the apple or something like that? like for example, if i trow an apple like a baseball, it will follow a parabola trajectory that agrees with relativity but if it falls form a tree straight to the ground, i dont see a parabola anywhere, so, how come?

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No one here is answering your actual question, which is whether there is a sense in GR in which the ground rises up to hit the apple. Everyone is answering in terms of Newtonian gravity, which, of course, GR replaces.

The answer is essentially "yes." In GR, inertial reference frames are those that are freely falling. So the apple is in an inertial reference frame, while the surface of the Earth is not (since an Earthbound observer with her feet on the ground is not freely falling). That observer is constantly accelerated away from her natural motion (a spacetime geodesic) by the constant upwards normal force of the Earth. She knows that she is not an inertial observer because she feels a force on her feet--- while the apple feels no forces.

So from the inertial reference frame of the apple, the answer is that the apple is at rest (it's inertial) and the surface of the Earth rapidly accelerates towards it until it collides with it, and thenceforward forever accelerates it away from natural motion at 9.8 m/s.

To answer the other question: An apple falling towards the Earth certainly executes a parabola, as observed by everyone on the surface of the Earth, except the one observer who is at rest relative to the tree. To anyone walking by, the motion of the apple is absolutely a parabola.

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A parabola trajectory is very visible if you throw an apple or a baseball in any direction other than straight up. An object on a parabola trajectory curves up, arcs over, then curves back down to hit the Earth.

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