Question

There is a $1\text{mm}$ thick layer of glycerine between a flat plate of area $100{\text{cm}}^2$ & a big fixed plate. If the coefficient of viscosity of glycerine is $1.0\text{kg/m-s}$, then how much force is required to move the plate with a velocity of $7\text{cm/s}$?

• A. $7\text{N}$
• B. $0.7\text{N}$
• C. $0.007\text{N}$
• D. None of the above

Answer 1

The correct option is B $0.7\text{N}$

From Newton's law of viscosity, force required
$F=\eta A\frac{dv}{dx}$
$\Rightarrow F=1\times 100\times {10}^{-4}\times \frac{7\times {10}^{-2}}{1\times {10}^{-3}}$
$\Rightarrow F=0.7\text{N}$