A common expression for hydrodynamic drag is a a force that goes $\propto -k v^2$.
Physically I have alwasys interpreted this as being related to the ram pressure $\propto \rho v^2$, i.e. for an object to move through a fluid at speed $v$, one needs to impart momentum $\propto v$ and kinetic energy $\propto v^2$ to air molecules for them to move out of the way.
Once the object reaches the speed of sound, however, it will be moving through the fluid faster the molecules get out of the way, hence causing a shock wave - the familar sonic boom.
How will air resistance change, if it will, in the supersonic regime?
