Let a point move in the system k (which moves with velocity v along the x-axis of the system K) according to the equation
where wξ and wη are constants.
It is required to find out the motion of the point relative to the system K. If we now introduce the system of equations in § 3 in the equation of motion of the point, we obtain
The law of parallelogram of velocities hold up to the first order of approximation. We can put
and
i.e., α is put equal to the angle between the velocities v, and w. Then we have—