Which parameter is used to express major head loss in the Darcy–Weisbach equation?

• A. Friction factor f
• B. Reynolds number Re
• C. Velocity V
• D. Pressure

Answer: (A)

Major head loss in a pipe comes from friction along the pipe walls, and it is captured by the friction factor in the Darcy–Weisbach relation. The head loss due to friction is h_f = f (L/D) (V^2 / 2g). In this expression, the friction factor f is the parameter that directly scales the loss for a given length, diameter, and flow velocity. It embodies how roughness and the flow regime (reflected in Re and surface texture) affect friction. The velocity term (V^2/2g) sets the basic energy head being lost, but the actual amount of loss for the pipe is governed by f. Reynolds number and velocity influence f and the kinetic term, respectively, but the quantity that expresses the major frictional head loss is f. Pressure relates to head through P/γ but the Darcy–Weisbach form expresses the loss explicitly via the friction factor, not pressure itself.