Mach number
When an object travels through any medium, then its Mach number is the ratio of the object's speed to the speed of sound in that medium. It is a dimensionless number, typically used to describe the speed of aircraft. Mach 1 is equal to the speed of sound, Mach 2 is twice that speed, etc. Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on its altitude and the atmospheric conditions. High speed flight can be classified in six categories:| Subsonic | M < 1 |
| Sonic | M = 1 |
| Supersonic | M > 1 |
| Transsonic | 0.8 < M < 1.3 |
| Hypersonic | 5 < M < 10 |
| Hypervelocity | M > 10 |
At transsonic speeds, the object itself is not moving at supersonic speeds, but air moving around the object may be, and this creates small shocks that may affect aircraft control.
When an aircraft exceeds Mach 1 (i.e. the sound barrier) a large pressure difference is created just in front of the aircraft. This pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape. It is this shock wave that causes the sonic boom heard as fast moving aircraft travels over head. A person flying inside the aircraft will usually not hear this.
At hypersonic speeds, frictional heating becomes very important.
The Mach Number (M) is the quotient of the inertial force (also referred to the aerodynamic force) and the elastic force.
The Mach number is named after Austrian physicist and philosopher Ernst Mach.