I am working on part (i). What is the resistive force?
Shear stress= μ*(du/dy) = μU/l = F/A
Since the force is up and down, it causes a shear stress, thus area is parallel to the force and thus it is the 4 faces B*L
→ F=μ*U*A/l = 4*μ*U*L*B/l
Where = viscosity
L=length of block
I'm not sure if that is the correct method. Can someone please verify?