## Quick Reference

The mass of a moving body as measured by an observer in the same frame of reference as the body. According to the special theory of relativity the mass *m* of a body moving at speed *v* is given by *m* = *m*_{0}/√(1 − *v*^{2}/*c*^{2}), where *m*_{0} is its rest mass and *c* is the speed of light. The relativistic mass therefore only differs significantly from the rest mass if its speed is a substantial fraction of the speed of light. If *v* = *c*/2, for example, the relativistic mass is 15% greater than the rest mass.

*Subjects:*
Physics.