The velocity u can then be thought of as the inferred velocity relative to the ship from Doppler shifts alone.

Unlike the relativistic addition formula, the velocity u is not the physical velocity of the object.

The velocity u can now be given a physical interpretation, although an unusual one.

The final velocity v of an object which starts with velocity u and then accelerates at constant acceleration a for a period of time is:

A child inside the train throws a baseball due east with a velocity u with respect to the train.

The limiting case applies when the velocity u is very small compared to c, the speed of light.

Additionally in classical mechanics, the three dimensional quantity for a particle of mass m moving with velocity u:

If a side-slip velocity u is introduced, the contact patch will be deformed.

Far away from the plate, for z , the velocity u approaches zero.

Further the velocity u oscillates with amplitude U far away from the wall, z .