Question

frame moving at constant velocity with inertial frame is also an inertial frame

Answer 1

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Because it is as if they are not moving and are still. Their relative velocity will be zero. It will be identical in the aspect of being a frame of reference, so if both measure any acceleration of an accelerating body moving past them, simultaneously, it will be identical. This question in a way brings concepts of special theory relativity and the general theory of relativity together.