Question

A plate of metal 100 cm${}^2$ in area rests on a layer of castor oil 2mm thick, whose coefficient of viscosity is 15.5 poise. The horizontal force required to move the plate with a uniform speed of $3cm{s}^{-1}$ is:

• A. 28,250 dyne
• B. 23,250 Newton
• C. 23,250 dyne
• D. 28,250 Newton

Answer 1

The correct option is C 23,250 dyne

Force = $\eta A\frac{dv}{dy}$

Now, $\frac{dv}{dy}=\frac{3cm/s}{\frac{2}{10}cm}=15s^{-1}$

So, force= $=100\times 15.5\times 15dyne$$=23250dyne$