When an object is moving at a very high velocity, particularly close to the speed of light, its mass is commonly referred to as relativistic mass. Relativistic mass is a concept derived from Einstein's theory of special relativity, which describes the behavior of objects moving at significant fractions of the speed of light.
According to special relativity, as an object's velocity approaches the speed of light (c), its mass appears to increase from the perspective of an observer at rest relative to the object. This increase in mass is known as relativistic mass. It implies that the object's inertia also increases, requiring more energy to accelerate it further.
However, it's important to note that the concept of relativistic mass has fallen out of favor among physicists, and the modern interpretation of mass in relativity is based on the object's rest mass. Rest mass, also known as invariant mass, is the mass of an object as measured by an observer in a frame of reference where the object is at rest. Rest mass remains constant regardless of the object's velocity and is considered the intrinsic mass of the object.
In modern physics, the concept of relativistic mass is generally avoided due to its potential for confusion. Instead, the focus is on the object's rest mass, which remains constant, while relativistic effects such as time dilation and length contraction occur as an object approaches the speed of light.