I am a little confused about mass in STR.
In the rest frame of an object, you (A) can measure the mass using its operational definition i.e. from let's say a spring. Now for an observer (B) wrt. whom this mass moves at v. How can he measure its inertial and relativistic masses, like through which experiment? Also when he measures this would he find it as increased compared to A or same (ie whether it would be inertial or relativistic?)
A similar question is here already. There the answers focus on the definition of mass etc. What I have asked specifically is how to measure it? For instance, in the rest frame a mass can be measured by springs etc. An experiment or method that can measure it another frame. So basically I am seeing a body zooming past me, how do I measure its mass?
Then, when we measure this mass what will we get? (Previous answers I think say it will be an increased mass but don't give reason as to why then relativistic mass is getting abandoned) since if definitions were not made or the person measuring this mass didnt know the definition, for him this mass would be the actual mass and he will note it as a mass without knowing the difference between inertial and relativistic.
The previous question does pertain to the latter part of my question but I am looking for answers to the first and a little clarity on the follow up which is the second part of ny question