Mach Number
The ratio of an object's speed to the local speed of sound (~343 m/s in dry air at 20 °C).
Definition
The Mach number (M) is a dimensionless quantity representing the ratio of the speed of an object to the speed of sound in the surrounding medium. Mach 1 equals the speed of sound. Objects moving below Mach 1 are subsonic; above Mach 1 is supersonic; above Mach 5 is hypersonic. Named after physicist Ernst Mach.
Formula
M = v / c, where c ≈ 343 m/s (dry air, 20 °C)
Examples
- Concorde cruised at Mach 2 (≈ 2,180 km/h).
- The SR-71 Blackbird reached Mach 3.3.
- A commercial airliner cruises at about Mach 0.85.