Question

There is a $1mm$ thick layer of glycerine between a flat plate of area $10c{m}^2$ & a big fixed plate. If the coefficient of viscosity of glycerine is $1.0kg/m-s$, then how much force is required to move the plate with a velocity of $7cm/s$ .

• A. $3.5N$
• B. $0.7N$
• C. $1.4N$
• D. None

Answer 1

The correct option is B $0.7N$

Using the relation $\tau =\frac{F}{A}=\mu \frac{du}{dy}$
where F = force is required to move the plate
$\tau$ = Shear stress
A = Area of plate
$\mu$ = coeffecient of viscosity of glycerine
y = gap
u = velocity
$\frac{F}{{10}^{-2}}=1\times \frac{0.07}{0.001}$.
Thus F = 0.7 N