Question

In the previous question, the net force exerted by fluid on moving plate is

Answer 1

F = \frac{\eta A v}{d} \quad \text{(opposing the motion)}.

Answer 2

For a plate sliding over a fluid layer of thickness $d$ with velocity $v$, the fluid exerts a shear stress given by

$$\tau = \eta\,\frac{v}{d},$$

where $\eta$ is the viscosity. Multiplying by the area $A$ of the plate gives the viscous force,

$$F = \eta\,A\,\frac{v}{d}.$$

This force acts opposite to the direction of motion.