Caption: An axiom of general relativity (presented by Albert Einstein (1879--1955) in 1915) is that there is NO way in principle to distinguish motion in a reference frame accelerated relative to an inertial frame with NO gravitational field and motion relative inertial frame in a uniform gravitational field.
The accelerated reference frame is a non-inertial frame, but it can always be converted to an inertial frame by invoking an inertial force which acts just like gravity.
The upshot is that there is an inertial frame in which to analyze systems where you want one.
Recall all physical laws in the observable universe are referenced to inertial frames, except general relativity which tells us what inertial frames are.
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