Question

why does mass rises of a moving body relative to a standing observer​

Answer 1

Answer:

Time slows down for an observer moving arbitrarily close to the speed of light because the subatomic particles that make up the atoms in the body of the observer are also limited to light speed.

Answer 2

Answer:

Mate ur question can be answered by the famous theory of relativity

which is E=mc²

But it's not actually mass but it's called

INERTIAL MASS