What they are calling the "relativistic mass" is really the relativistic four-momentum. Classical momentum (p=mv) is not conserved for velocities close to c (the speed of light). Relativistic momentum ( E/c + p) is conserved.Originally Posted by M21Sniper
What they are discussing are the conserved quantities for Lorentz transforms. Lorentz transforms are used to translate what one observer sees relative to another observer moving at velocities close to c.
They are not discussing particle physics, although particle physics does make use of the Lorentz transform.