First slide

Viscosity

Question

A small sphere of mass 6 mg is falling through a viscous liquid of specific gravity 0.8 with constant velocity of 2 cm/s. If the specific gravity of the material of the sphere is 2.4, the viscous force acting on it is

Moderate

A

6 dyne
B

4 dyne
C

12 dyne
D

information is not sufficient to find the viscous drag

Solution

At terminal velocity,
Viscous force $F_{D} = m g - \frac{m}{σ} . ρ . g = m g (1 - \frac{ρ}{σ})$
$= 6 \times 10^{- 6} \times 10 \times (1 - \frac{0.8}{2.4}) N$
$= 4 \times 10^{- 5} N = 4 dyne$

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