Application of Poisueille’s Law

• Water level is 12 m above ground. Hose nozzle is 2.0 m above ground and has a radius of 0.01 m and length 50 meters. Viscosity of water is h=0.0010 pascal-sec.

• : What is flow rate with nozzle removed?

• : DP = rgDy = 98,000 Pa

• Q = p R4 DP / (8 h L)

• = 7.7 x 10-3 m3/s = 7.7 l/s

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Here we look at the effects of viscous friction on the example used in an earlier slide.

Firemen use big hoses for two reasons: 1) To get a big flow rate, Q, for a given pressure at the hydrant, and 2) to make the reduction in pressure between hydrant and nozzle as small as possible (Poisuelli’s Law) so that the water will leave the nozzle with greater speed (Bernoulli’s equation) and, therefore, travel further (projectile motion).