Relativity......

Question

If you can't determine if an object is in motion due to the theory of relativity, why can't you just measure the mass, if mass increases with speed and tell if it is moving?

Answer 1

If you want to determine whether an object is accelerating, that is easy,

because acceleration is not relative.

Only velocity is relative to a frame of reference.

In a sense you are right, you can measure mass to determine velocity.

But (unlike acceleration) mass is not absolute -- it is relative to a frame of reference.

By measuring mass WITH RESPECT TO a frame of reference, you can indeed determine its

speed WITH RESPECT TO the same frame of reference.