why does the mass depends upon the velocity but charge does not?
Answers
n the first place mass doesn't depend on velocity. This is an idea that Einstein briefly considered and then soon rejected. It is possible to interpret equations of special relativity as if they are consistent with mass increasing with velocity, but it is not a necessary interpretation, and it makes the mathematics less elegant.
However, for the sake of discussion, let's say that mass does increase with velocity. Mass is not a conserved quantity as far as we can tell, as it is just one way in which energy is stored. That means that there is no fundamental problem if the mass of stuff in some part of the universe would change. OTOH, electrical charge must be an absolutely conserved quantity for quantum electrodynamics to work as well as it does. That means that if electrical charge were a function of speed or velocity, there would be a real problem with the most successful theory ever formulated.