In general, for isolated systems and single observers, relativistic mass is conserved (each observer sees it constant over time), but is not invariant (that is, different observers see different values).
https://en.wikipedia.org/wiki/Mass_in_special_relativity#:~:text=In%20general%2C%20for%20isolated%20systems,different%20observers%20see%20different%20values).
Mass(m) of a rigid body is same everywhere, but its weight(mg) may be variable at different positions.
If we throw upward a balloon filled with air and its mouth tied, the balloon comes downward and stay on the ground if there is no wind there. Because balloon has mass and its weight.
Shinsuke Hamaji, Thanks for your answer.
Let us go step by step.
Does the mass defined in the kinetic energy of a moving rigid body vary?
Rigid body: Kinetic energy and Mass of rigid body. What is rigid body? and does its mass vary?
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