It is said that the aircraft is flying at Mach 1 if its speed is equal to the speed of sound in air (which is 332 m/s or 1195 km/hr or 717 miles/hour.) An aircraft flying at Mach 2 is flying at twice the speed of sound in air, etc. Mach numbers are named after Ernst Mach (1838-1916), an Austrian philosopher and physicist. The term Mach number came into use in 1929.
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'There must be no barriers for freedom of inquiry. There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors.'
J. Robert Oppenheimer