in 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 = E₀ + ¹/₂mv², where the value of the “rest energy” E₀ was undetermined. In special relativity the corresponding formulas for p and E, respectively, are p = mv/ √((1 - v²/c²)) and E = mc²/ √((1 - v²/c²)) , 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 - v²/c²)) . Then, for all velocities we have the simple formulas p = m_Rv and E = m_Rc² 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.