Special relativityAccording to the theory of special relativity, the frame of the ship has a different clock rate and distance measure, and the notion of simultaneity in the direction of motion is altered, so the addition law for velocities is changed. This change is not noticeable at low velocities but as the velocity increases towards the speed of light it becomes important. The addition law is also called a composition law for velocities. For collinear motions, the velocity of the fly relative to the shore is given by The composition formula can take an algebraically equivalent form, which can be easily derived by using only the principle of constancy of the speed of light:[2] https://en.wikipedia.org/wiki/Velocity-addition_formula Did you mean this?