Mach number is defined as which of the following?

• 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 density of the gas
• C. The ratio of the gas velocity to the viscosity of the gas
• D. The ratio of the gas velocity to the velocity of light

Answer: (A)

Mach number compares how fast the flow is moving to how fast sound travels through the same gas. It is defined as V divided by a, where V is the flow velocity and a is the local speed of sound in the gas under current conditions. The speed of sound depends on temperature and gas properties, so Mach tells you whether the flow is subsonic, sonic, or supersonic.

This exactly matches the idea of the ratio to the speed of sound, making it the correct choice. The other ratios (to density, to viscosity, or to the speed of light) describe completely different physical relationships and do not characterize how pressure disturbances propagate in the gas.