What is a Mach (Ma) number?

• A. The ratio of the gas velocity to the velocity of sound in the gas at prevailing conditions
• B. The ratio of the gas velocity to the velocity of light
• C. The ratio of the speed of sound to the gas velocity
• D. The ratio of the gas density to the velocity of sound

Answer: (A)

Mach number tells you how fast the gas is moving compared with the speed of sound in that gas. It’s defined as M = V / a, where V is the gas velocity and a is the local speed of sound (which depends on temperature and the gas’s properties). This makes the Mach number a dimensionless measure that indicates whether the flow is subsonic, sonic, or supersonic.

So the correct description is the ratio of the gas velocity to the speed of sound in the gas under the prevailing conditions. The other ideas don’t match Mach number: using the speed of light isn’t relevant to compressible flow in typical engineering contexts, the inverse ratio would describe 1/M rather than M, and a density-to-sound-speed ratio has no direct interpretation as Mach number.