 relativistic mass

the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases.
* * *
▪ physicsin the special theory of relativity, the mass that is assigned to a body in motion. In physical theories prior to special relativity, the momentum p and energy E assigned to a body of mass m and velocity v were given by the formulas p = mv and E = E_{0} + ^{1}/_{2}mv^{2}, where the value of the “rest energy” E_{0} was undetermined. In special relativity the corresponding formulas for p and E, respectively, are p = mv/ √((1  v^{2}/c^{2})) and E = mc^{2}/ √((1  v^{2}/c^{2})) , where c equals the speed of light (300,000 kilometres [186,000 miles] per second) and m is the “rest mass” of the body (i.e., its mass as determined when the body is at rest). It is convenient for certain purposes to define the relativistic mass m_{R} of a body by the formula m_{R} = m/ √((1  v^{2}/c^{2})) . Then, for all velocities we have the simple formulas p = m_{R}v and E = m_{R}c^{2} for the momentum and energy of a body. The relativistic mass m_{R} becomes infinite as the velocity of the body approaches the speed of light, so, even if large momentum and energy are arbitrarily supplied to a body, its velocity always remains less than c.* * *
Universalium. 2010.