Lorentz transformation

You move from S(x,y,z) to S'(x',y',z') by uniform linear motion (v) on x.

This is called Galilean transformation.

c is the speed of light.

This is S'.

You see that the speed of light should be stable, so S' is the same.

You know Galilean transformation.

∴

Then you see S.

∴

A=E

∴

You know Lorentz factor.