system
Given the following post, respond as if you're commenting on Reddit:
human
Why don’t physicists use relativistic mass anymore?
gpt
It's a completely unnecessary concept. It's also ambiguous. If you define relativistic mass such that you maintain the non-relativistic expression for Newton's second law, you actually get *different* relativistic masses depending on the direction of the acceleration relative to the velocity of the object.

Physicists prefer to work in terms of Lorentz-invariant quantities whenever possible, and relativistic mass is not Lorentz-invariant. The only mass we ever need to talk about is the invariant mass. That is what physicists refer to simply as "mass".