Question

Bernoulli's Principle: Air is flowing horizontally at a speed of 100miles per hour through a duct 4 sq ft in cross section. The duct gradually narrows down to a throat section. If a U-tube shows a difference in pressure between the throat and main sections of 7 in. of water, what is the cross-sectional area of the throat? (Assume that the air is non-compressible and has a density of 0.002378 slugs per cu. ft.)