In external flow, the flow resistance causes an unknown pressure drop, Δp, which causes an unknown head loss, hf, where the head loss is due to a conversion of kinetic energy to heat, which is...

In external flow, the flow resistance causes an unknown pressure drop, Δp, which causes an unknown head loss, hf, where the head loss is due to a conversion of kinetic energy to heat, which is modeled/displayed in the integral form of the energy equation. However, although the head loss, hf causes the drag force, FD, the head loss is not actually determined in the design of external flow around an object. Explain why the flow resistance is modeled as a drag force (as opposed to a minor head loss) in external flows.