Mach Number

The ratio of the speed of a body or flow of a fluid to the local speed of sound in the fluid.

M = Mach Number
U = characteristic velocity, ms-1
c = speed of sound, ms-1
The first aircraft in the world to break the sound barrier was the Bell X1. On the 14 October 1947 it reached 670mph at 42000feet (Mach 1.015). The X1 was launched by a Boeing B29 at 30000 feet, once released the rocket motor on the X1 was fired and it would begin it's ascent.

If the Mach number is less than 1, the flow is called subsonic and local disturbances can propagate ahead of the flow. If the Mach number is greater than 1, the flow is called supersonic and disturbances cannot propagate ahead of the flow with the result that shock waves form.

In the atmosphere, the speed of sound varies with temperature and atmospheric pressure, hence, so does mach number.

See also: Sound, Subsonic, Supersonic, Velocity.

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Subjects: Physics

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