A flat plate of area $10\,cm^2$ is separated from a large plate by a layer of glycerine $1\, mm$ thick. If the coefficient of viscosity of glycerine is $20$ poise, the force required to keep the plate moving with a velocity of $1\,cm/sec$ is .......... $dyne$
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${F_v}\, = \,\eta A\,\frac{{dv}}{{dy}}\, = \,20\, \times \,10\, \times \,\frac{1}{{{{10}^{ - 1}}}}\, = \,2000\,{\rm{dyne}}$
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