Question

A uniform plate of area $10c{m}^2$ is placed on a horizontal surface coated with an oil layer of thickness 5 mm. The force that must be applied horizontally on the plate to move it with a speed of 5 cm/s is

• A. $3\times {10}^{-2}N$
• B. $2.5\times {10}^{-2}N$
• C. $2\times {10}^{-2}N$
• D. $1.5\times {10}^{-2}N$

Answer 1

The correct option is A $3\times {10}^{-2}N$

$\begin{array}{c}F=\eta A\frac{dv}{dt}\\ F=\eta \left(\frac{10}{10000}\right).\frac{\left(\frac{5}{100}-0\right)}{\left(5\times {10}^{-3}\right)}\\ F=\eta \times \frac{10}{10000}\times \frac{5}{100\times 5\times {10}^{-3}}\\ F=\frac{\eta }{10}\end{array}$

Then,

Option $A$ is correct answer.