## On Velocity Addition (4)

Similarly, under Special Relativity, we have another set of definitions and assumptions for space and time.

Based on this set of definitions and assumptions, an unique velocity addition rule can also be obtained.

Letâ€™s go back to our spaceship example. Whether we are on Earth or in one of the spaceships, we can always use the same velocity addition formula.

As Newtonian system and Relativitic system have incompatible assumptions for space and time, their velocity addition rules are also incompatible.

How do we know which is which?

By their mathematical form.

Any velocity addition either conforms to one or another, never both.

What is v1*v2?

Nothing, at least it has no physical meaning.