On Velocity Addition (1)
About velocity addition, there are several common misunderstandings.
One misunderstanding is that velocity addition rule applies to some reference frames, but not to others.
Specifically, in our spaceship example, the relativistic transformation formula can be used when you are in one of the spaceships, but not when you are on Earth.
Another misunderstanding is that the c+v and c-v in Einstein's paper is not Galileo Transformation.
All these misunderstandings have the same root cause:
We know there are velocity addition rules, but we don’t know why they are there.
So here we will start with the very basics.