v에따라 변하는것은 momentum

called "relativistic momentum"

저 식을 쉽게 이해할려고 해석을한게 질량이 변한다는것처럼 말하는것임(실제 질량은 절대 변하지않음)

실제 속도에따라 증가하는것은 질량이아닌 운동량.

못믿을사람들을 위해 .. 위키에 relativistic momentum 내용 중 일부(해석은 알아서 해보길)

It is not good to introduce the concept of the mass  of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the ’rest mass’ m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion.

Many contemporary authors such as Taylor and Wheeler avoid using the concept of relativistic mass altogether:

"The concept of "relativistic mass" is subject to misunderstanding. That's why we don't use it. First, it applies the name mass - belonging to the magnitude of a 4-vector - to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself."^[6]

위 영문 출처 : https://en.wikipedia.org/wiki/Mass_in_special_relativity