Rest mass

In special relativity, rest mass is an obsolete term used to describe what is today simply referred to as mass. Relativistic mass m_r (formerly called "mass", but now called energy) increases with velocity (v), and the rest mass m is the inertial mass at v=0, which corresponds to the classical notion of mass.

When an object is travelling at a velocity v relatively to an inertial reference frame, then the relativistic mass increases according to:

It should be noted that the rest mass is one of the invariant scalar quantities of special relativity.

The terms rest mass and relativistic mass can be still be found in elementary textbooks and popularisations of relativity, but their use is depreciated in graduate-level physics.