Mach number

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An F/A-18 Hornet creating a vapor cone at transonic speed just before reaching the speed of sound

A mach number is a number that describes speed. It is a measure of the speed of an object compared to the speed of sound.[1] Mach number is given as a percentage compared to the local speed of sound. For example, half the speed of sound = Mach 0.5.

${\displaystyle M=v/a}$

Where v is the speed of the object, and a is the speed of sound.[2]

${\displaystyle \mathrm {M} ={\frac {u}{c}},}$

It is named after the Austrian physicist Ernst Mach. When writing a speed as a Mach number, the number comes after "Mach."[2] For example, the Concorde airplane could fly at Mach 2, and a satellite re-enters the Earth's atmosphere at about Mach 25.[3]

References

1. Young, Donald F.; Bruce R. Munson; Theodore H. Okiishi; Wade W. Huebsch (2010). A Brief Introduction to Fluid Mechanics (5 ed.). John Wiley & Sons. p. 95. ISBN 978-0-470-59679-1.
2. Graebel, W.P. (2001). Engineering Fluid Mechanics. Taylor & Francis. p. 16. ISBN 978-1-56032-733-2.
3. Bodie, Warren M., The Lockheed P-38 Lightning, Widewing Publications ISBN 0-9629359-0-5.