Correct choice is (b) \(\vec{F}=-\mu\frac{\vec{u}-\overrightarrow{u_{wall}}}{\Delta y}\times area\)
The explanation: Shear force is a product of shear stress and area. Shear stress is defined by Newton’s law of viscosity. For moving walls, the relative velocity \(\vec{u}-\overrightarrow{u_{wall}}\) should be taken. Therefore, the shear force is given by \(\vec{F}=-\mu\frac{\vec{u}-\overrightarrow{u_{wall}}}{\Delta y}\times area\).