To conclude this section, we should say something about the assumption that the trading planets lie in the same inertial frame.
N = number of years taken to travel from Earth to Trantor (or vice versa), as measured by an observer in the Earth-Trantor inertial frame.
First Fundamental Theorem of Interstellar Trade: When trade takes place between two planets in a common inertial frame, the interest costs on goods in transit should be calculated using time measured by clocks in the common frame and not by clocks in the frames of trading spacecraft.
As seen from our point of view, the object work fly in a straight line as in an inertial frame
, but it will deviate in a direction that is perpendicular to its velocity.
On the contrary, in the Lorentz transformations, given any inertial reference frame (K', K, or any other inertial frame
), there is c' = c and, hence, the velocity of light in the inertial frame
K, being measured by the observers located in the inertial frames
K' and K is always the same.
More on inertial frames
and introduction to the equivalence principle lead us into inertial forces and, finally, the bending of light by gravity.
More advanced applications - including gravitational orbits, rigid body dynamics and mechanics in rotating frames - are deferred until after the limitations of Newton's inertial frames
have been highlighted through an exposition of Einstein's Special Relativity.
The theory of special relativity of Albert Einstein is essentially based on the constancy of the velocity of light in all inertial frames of reference.
The relativity principle states that all inertial frames are equivalent for describing the laws of physics.
These just alluded situations should be appreciated by consideration (prevalently or even asymptotically) of Einstein's posulate of relativity, which states  that the inertial frames
of references are equivalent to each other, and they cannot be distinguished by means of investigation of physical phenomena.