# The Unit used to Measure the Supersonic Speed to

- knot
- Richter
- hertz
- Mach

**Option 4 is the right answer.**

In fluid dynamics, the Mach is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.

The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature and pressure. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The medium can be a gas or a fluid. The boundary can be traveling in the medium, but can be stationary while the medium flows along it.