### Question

What speed defines Mach 1? and why?
Asked by: Anthony Matarazzo

Mach number is a common 'ratio' unit of speed when one is talking about aircrafts. By definition, Mach number is a ratio of the speed of a body (aircraft) to the speed of sound in the undisturbed medium through which the body is traveling. 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.
Answered by: Anton Skorucak, M.S. Physics, PhysLink.com Creator

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'One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike - and yet it is the most precious thing we have.'

Albert Einstein
(1879-1955)