A flat plate of area $0.1 \,m ^2$ is placed on a flat surface and is separated from it by a film of oil $10^{-5} \,m$ thick whose coefficient of viscosity is $1.5 N \,sm s ^{-2}$. The force required to cause the plate to slide on the surface at constant speed of $1 \,mm s ^{-1}$ is ............ $N$
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(b)
$F=\eta A \frac{v}{l}$
Substituting values,
$=1.5 \times 0.1 \times \frac{1 \times 10^{-3}}{10^{-5}}$
$=15 \,N$
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