A student performs a flow experiment with Bingham Plastic under fully developed laminar flow conditions in a tube of radius 0.01 m with a pressure drop (ΔP) of 10 kPa over tube length (L) of 1.0 m. The velocity profile is flat for rc and parabolic for r≥r_c, as shown in the figure.

Consider r and x as the radial and axial directions, and the shear stress is finite as r approaches zero. A force balance results in the following equation

where τ_rx is the shear stress. If r_c is 0.001 m, then the magnitude of yield stress for this Bingham Plastic (in Pa) is