Question

A plate of area `100 cm^(2)` is placed on the upper surface of castor oil, `2mm` thick. Taking the coefficient of viscosity to be `15.5` poise, calculate the horizontal force neccesary to move the plate with a velocity `3cm^(-1)`.

Answer 1

The (horizontal tangential) viscous force is given by
`F=-etaA(dv)/(dy)`
Given `eta=15.5` poise, `A=100cm^(2)`
`(dv)/(dx)=(v_(2)-v_(1))/(x_(2)-x_(1))=((0-3)cm//s)/((2-0)mm) =-3//0.2=-15 s^(-1)`
`F=15.5xx100xx(-15)=2.235xx10^(4)dyn=0.2325N`