With all else equal, under which of the following conditions would the drop in pressure occurring while a fluid flows through a tube be greatest?

Tube Diameter Fluid Viscosity
A. A Small Low
B. B Large Low
C. C Large High
D. D Small High

a. A; Small; Low
b. B; Large; Low
c. C; Large; High
d. D; Small; High

ANS: D
Available energy decreases because frictional forces oppose fluid flow. Frictional resistance to flow exists both within the fluid itself (viscosity) and between the fluid and the tube wall. In general, the greater the viscosity of the fluid and the smaller the cross-sectional area of the tube, the greater is the drop in pressure along the tube.