## About Mass (2)

Putting Newton's second law of motion

into two reference frames, we get:

*The second law of motion under Special Relativity*

Under Special Relativity, the length, time, and mass all change along with their reference frames, so the equation above cannot be simplified any further.

*The second law of motion under Newtonian system*

In the Newtonian system, the length, time, and mass are the same in all reference frames, so

becomes:

By Integration, we get:

This is the normal velocity transformation formula.

So, in the Newtonian system, the first postulate and the velocity transformation formula are the same thing.