If,in a certain reference frame,an object with no unbalanced forces on it keeps moving with constant velocity,then that frame must be an inertial reference frame.

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Quizplus · Accepted Answer

This statement is one of the postulates of the theory of special relativity, which defines an inertial reference frame as one in which an object with no unbalanced forces on it will move with constant velocity. Therefore, if an object is observed to move with constant velocity in a certain reference frame, it must be an inertial reference frame.

If,in a Certain Reference Frame,an Object with No Unbalanced Forces