shear flow

Problem 568
Show that the shearing stress developed at the neutral axis of a beam with circular cross section is τ = (4/3)(V / π r²). Assume that the shearing stress is uniformly distributed across the neutral axis.
 

Problem 567
A timber beam 80 mm wide by 160 mm high is subjected to a vertical shear V = 40 kN. Determine the shearing stress developed at layers 20 mm apart from the top to bottom of the section.