The Mach number is defined as the ratio of the fluid velocity to the speed of sound in the medium.

• A. The ratio of the speed of sound to the fluid velocity
• B. The ratio of the fluid velocity to the speed of sound in the medium
• C. The ratio of the fluid velocity to the speed of light in vacuum
• D. The ratio of the speed of sound to the density of the fluid

Answer: (B)

Mach number is a dimensionless quantity that compares how fast the fluid is moving to how fast sound travels in the same medium. It is defined as M = v / a, where v is the fluid velocity and a is the speed of sound in the medium. This ratio determines whether compressibility and wave propagation effects are important: subsonic flows have M < 1, sonic flow is around M ≈ 1, and supersonic flows have M > 1, often leading to shocks and strong compression. The speed of sound depends on the medium and its temperature, for example in an ideal gas a ≈ sqrt(γRT), so the Mach number reflects flow speed relative to the local acoustic speed, not just the raw velocity alone. The alternative ratios do not describe how quickly disturbances propagate relative to the flow: swapping the ratio inverts the quantity, the speed of light is unrelated to sound propagation in a fluid, and density is not the characteristic speed for pressure disturbances.