First slide

Viscous flow/Laminar flow

Question

A small metal ball of diameter 4 mm and density 10.5 g/cm³ in dropped in glycerin of density 1.5 g/cm³ . The ball attains a terminal velocity of 8 cm s^-1. The coefficient of
viscosity of glycerin is

Easy

A

4.9 poise
B

9.8 poise
C

98 poise
D

980 poise

Solution

$\begin{array}{l} v_{0} = \frac{2}{9} \frac{r^{2} (ρ - ρ^{'}) g}{η} or η = \frac{2 r^{2} (ρ - ρ^{'}) g}{9 v_{0}} \\ = \frac{2 \times {0.2}^{2} \times (10.5 - 1.5) \times 980}{9 \times 8} poise = 9.8 poise \end{array}$

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