In the context of special relativity, the concept of relativistic mass is no longer commonly used or preferred. Instead, physicists generally refer to the relativistic effects in terms of energy, momentum, and velocity.
In special relativity, the energy and momentum of an object depend on its velocity relative to an observer. As an object's velocity approaches the speed of light, its energy and momentum increase without bound, and it becomes more difficult to accelerate the object further.
The relativistic mass, denoted as m_rel, is an older concept that was used to describe the increase in mass of an object as it approached the speed of light. It was defined as the ratio of an object's momentum to its velocity:
m_rel = m_0 / sqrt(1 - (v^2/c^2))
where m_0 is the rest mass (the mass of the object at rest), v is the velocity of the object, and c is the speed of light.
However, this concept of relativistic mass is not commonly used in modern physics. Instead, physicists now emphasize the concept of rest mass (m_0) and refer to the increase in energy and momentum of an object as it approaches the speed of light.
Therefore, it is more appropriate to discuss the energy and momentum of an object in the context of relativistic effects rather than the relativistic mass.