According to classical mechanics, the mass of an object does not change with its velocity. This concept is known as the principle of invariant mass or rest mass. In other words, the mass of a body remains constant regardless of its speed or motion through space.
This principle holds true as long as we consider classical mechanics and velocities much lower than the speed of light. However, when we delve into the realm of relativistic physics, specifically in Einstein's theory of special relativity, things change.
In special relativity, the concept of mass becomes more nuanced. Instead of using the term "mass," physicists prefer to use the concept of "relativistic mass" or "invariant mass." According to this theory, as an object's velocity approaches the speed of light, its relativistic mass appears to increase.
The increase in relativistic mass is a result of the interplay between energy and momentum. As an object accelerates to higher velocities, its kinetic energy increases. Since mass and energy are related (as described by Einstein's famous equation E=mc²), an increase in kinetic energy can be thought of as an increase in the object's effective mass.
However, it's important to note that even though the concept of relativistic mass is used in some contexts, it's not the preferred way to describe the behavior of objects in modern physics. Instead, physicists tend to focus on the concept of invariant mass, which remains constant regardless of the velocity of an object.