Am 18.10.2015 um 16:29 schrieb Stephen:

>> Also, think of this: If you throw an apple, it moves through space in a
>> /straight/ line - any apparent curvature of its trajectory is due to
>> Earth's surface constantly accelerating upwards. Not towards the apple
>> but away from Earth's center. In a lump of space that's busy shrinking.
>>
> 
> If you say so. But consider using another inertial frame of reference.
> One that contains both the Earth and the apple. Which object, the Earth
> or the apple, travels a greater distance in the same time frame?
> I hazard a guess that it will be the apple that has the greater velocity
> and thus acceleration.

Whatever frame of reference you choose - if it is truly inertial, you'll
always find the apple (while airborne) moving at constant velocity (or
remaining at rest, which is a special case thereof), and Earth's surface
accelerating radially from its center at 9.81 m/s^2. (As long as you
ignore aerodynamic drag and remain sufficiently close to Earth's surface
and the apple, that is.)

(Hint: What makes a frame of reference inertial? The fact that it
doesn't accelerate. How can you test it? By releasing some item with an
initial velocity of v=0 relative to the frame of reference, and
verifying that the item remains stationary. If you like, do the
experiment inside an elevator cabin with no windows.)

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