According to classical mechanics, mass is considered to be an invariant property of an object and is independent of its velocity. This concept is known as "rest mass" or "invariant mass." In classical mechanics, mass is an intrinsic property of an object that remains constant regardless of its motion.
However, in the context of relativistic mechanics, specifically in Einstein's theory of special relativity, mass is not considered a fundamental quantity. Instead, the relativistic mass concept was introduced to account for the increase in an object's apparent mass as its velocity approaches the speed of light. This concept is sometimes referred to as "relativistic mass" or "effective mass."
In special relativity, the relativistic mass (m_r) of an object moving with a velocity (v) is related to its rest mass (m_0) by the equation:
m_r = m_0 / sqrt(1 - v^2/c^2),
where c is the speed of light in a vacuum. As the velocity of an object approaches the speed of light (v → c), the denominator approaches zero, leading to an apparent increase in the object's mass.
It's worth noting that the concept of relativistic mass is not commonly used in modern physics, and the focus is typically on the rest mass (invariant mass) of particles. In relativistic equations, it is more common to work with energy and momentum rather than mass. The rest mass is considered the fundamental property of a particle, and it remains constant regardless of its velocity.