Relativistic mass

Mass in special relativity incorporates the general understandings from the concept of mass–energy equivalence. Added to this concept is an additional complication resulting from the fact that mass is defined in two different ways in special relativity: one way defines mass ("rest mass" or "invariant mass") as an invariant quantity which is the same for all observers in all reference frames; in the other definition, the measure of mass ("relativistic mass") is dependent on the velocity of the observer.

The term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object. The invariant mass is another name for the rest mass of single particles.

Relativistic mass can be expressed as:

`m' =  m / sqrt(1 − v^2/c^2)`

Where:-

m = the proper mass (the mass of the object in its rest frame),

 v = the relative velocity between the observer and the moving object
 c = the speed of light