Mach number Mach number In fluid dynamics, the Mach number (M or Ma) (; ) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound. where: By definition, at Mach1, the local flow velocity is equal to the speed of sound. At Mach0.65, is 65% of the speed of sound (subsonic), and, at Mach1.35, is 35% faster than the speed of sound (supersonic). The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature. The Mach number is primarily used to determine
Ernst Mach Mach are attributed a number of principles that distill his ideal of physical theorisation—what is now called "Machian physics": The last is singled out, particularly by Albert Einstein, as "the" Mach's principle. Einstein cited it as one of the three principles underlying general relativity. In 1930, he stated that "it is justified to consider Mach as the precursor of the general theory of relativity", though Mach, before his death, would apparently reject Einstein's theory. Einstein was aware that his theories did not fulfill all Mach's principles, and no subsequent theory has either, despite considerable effort. According to Alexander Riegler, Ernst