Question

Two flat plates are separated by a layer of 4mm thick liquid. The upper plate is moved by force of 400 dynes with a velocity of 10 cm/s. If the area of the upper plate is 5cm${}^2$, the coefficient of viscosity of the liquid is:

• A. 3.2 poise
• B. 1.6 poise
• C. 32 poise
• D. 16 poise

Answer 1

The correct option is A 3.2 poise

$\frac{dv}{dy}=\frac{10\times 10}{4}{s}^{-1}=25s^{-1}$
So, force = $\eta A\frac{dv}{dy}\Rightarrow 400dyne=\eta \times 5\times 25$
$\Rightarrow \eta =3.2poise$